Lower memory consumption by streaming out request data

Prior to this change, the `UrlConnectionHttpClient` implementation of
`SdkHttpClient` would unintentionally cause **the entire `RequestBody`**
to be loaded into memory before being sent, even if the body had been
made using `software.amazon.awssdk.core.sync.RequestBody.fromInputStream()`
and thus was a stream of known content-length. Even worse, as the buffering
`ByteArrayInputStream` received the request, it would have to repeatedly
grow capacity by allocating new arrays and copying the data over, so peak
memory usage would be 1.5x the size of the request data. For instance,
streaming 1 GB of data to S3 with `putObject` would lead to a peak memory
allocation of 1.5 GB. This large allocation of memory can be avoided by
instead *streaming* the request data as it is sent.

`java.net.HttpURLConnection` (the core Java class used by
`UrlConnectionHttpClient` to implement `SdkHttpClient`) supports streaming
the request, but will only do it if `setFixedLengthStreamingMode()` or
`setChunkedStreamingMode()` has been called - this is true even if the
`Content-Length` header has already been set. You can see this in the
implementation of `sun.net.www.protocol.http.HttpURLConnection` where
`streaming()` must be true to avoid **the whole request** being read into
a `PosterOutputStream` (a simple extension of `ByteArrayInputStream`)
before being sent:

https://github.com/openjdk/jdk/blob/da75f3c4ad5bdf25167a3ed80e51f567ab3dbd01/src/java.base/share/classes/sun/net/www/protocol/http/HttpURLConnection.java#L1373-L1394

If `Content-Length` is known, we're actually free to call
`setFixedLengthStreamingMode()` on the connection to activate streaming,
the only question is where & when to do it! Unfortunately
`software.amazon.awssdk.http.urlconnection.UrlConnectionFactory`, which
provides a way for end-users of the AWS SDK to customise the creation &
configuration of the `HttpURLConnection`, _doesn't have access to the
request or it's `Content-Length`, so can't call
`setFixedLengthStreamingMode()`. In any case, this behaviour probably
_always_ makes sense, so it doesn't make sense to leave it to users to
customise it. That leaves the best place to do this being the
`createAndConfigureConnection()` method, where the newly created
`HttpURLConnection` is available and the `Content-Length` can be read if
it's present.

Consequently, I've updated `createAndConfigureConnection()` to call
`setFixedLengthStreamingMode` if the `Content-Length` is not null, which
is similar behaviour to the `ApacheHttpClient` SDK client:

#2848 (comment)
https://github.com/aws/aws-sdk-java-v2/blob/db0b45a0cd29603a1ea95b5e5f3d243755720309/http-clients/apache-client/src/main/java/software/amazon/awssdk/http/apache/internal/impl/ApacheHttpRequestFactory.java#L139-L160

At present, the only way to workaround this issue without a change to the
AWS SDK is to create a custom `SdkHttpClient` that wraps
`UrlConnectionHttpClient` and overrides `prepareRequest()`. As the
`connection` field is not visible in the `ExecutableHttpRequest` object
(it's defined on the private
`software.amazon.awssdk.http.urlconnection.UrlConnectionHttpClient.RequestCallable`
class), it's necessary to use reflection to extract the `connection` field
before operating on it. An example of this in Scala code can be seen here:

guardian/ophan-geoip-db-refresher@e2fc370#diff-5780ade773d1bdf8dd5dea40283e6fec88bacdb30c20c5b39955da1faa8e3ade

Running the `ophan-geoip-db-refresher` code locally, the heap required is:

* 560 MB (`-Xmx560m`) - _without_ the workaround, request fully in memory
*  32 MB - with the workaround on, and so streaming enabled

Author: rtyley

.changes/next-release/feature-URLConnectionHttpClient-a1e8182.json

@@ -0,0 +1,6 @@
+{
+    "category": "URL Connection Http Client", 
+    "contributor": "rtyley", 
+    "type": "feature", 
+    "description": "Lower memory consumption for HTTP requests by enabling fixed-length streaming mode."
+}

http-clients/url-connection-client/src/main/java/software/amazon/awssdk/http/urlconnection/UrlConnectionHttpClient.java

@@ -15,6 +15,7 @@
 
 package software.amazon.awssdk.http.urlconnection;
 
+import static software.amazon.awssdk.http.Header.CONTENT_LENGTH;
 import static software.amazon.awssdk.http.HttpStatusFamily.CLIENT_ERROR;
 import static software.amazon.awssdk.http.HttpStatusFamily.SERVER_ERROR;
 import static software.amazon.awssdk.utils.FunctionalUtils.invokeSafely;
@@ -140,6 +141,9 @@ private HttpURLConnection createAndConfigureConnection(HttpExecuteRequest reques
         // See: https://github.com/aws/aws-sdk-java-v2/issues/975
         connection.setInstanceFollowRedirects(false);
 
+        request.httpRequest().firstMatchingHeader(CONTENT_LENGTH).map(Long::parseLong)
+               .ifPresent(connection::setFixedLengthStreamingMode);
+
         return connection;
     }