Include event type in visitor name

[dagnir]

Description

This commit fixes event visitor method generation by including the event type
in the visitor name. Note that event type here is actually the member name in
the encompassing eventstream structure, not the Java class of the event.

For example, in the example below, "Foo" is the event type.

"MyEventStream": {
   "type": "structure",
   "eventstream": true,
   "members": {
       "Foo": {
           "shape": "Bar"
       }
   }
}

To ensure that this change is backwards compatible with existing services that
have event streams (Transcribe Streaming and Kinesis), we also include a
customization to supress this behavior keep the method name as "visit".

Note that by sheer luck, the existing consumer method setters, e.g.

Builder onSubscribeToShardEvent(Consumer<SubscribeToShardEvent>);

are unaffected by this change. See an example here.
The previous generation scheme for these methods is "on{Generation Java class
name for event shape}"; now it's "on{event type}". However, both Transcribe and
Kinesis use the shape name as the member name (or vise versa?) in their current
iteration of their event streams.

For example, in Kinesis, SubscribeToShard's output stream is defined as

"SubscribeToShardEventStream":{
  "type":"structure",
  "required":["SubscribeToShardEvent"],
  "members":{
    "SubscribeToShardEvent":{"shape":"SubscribeToShardEvent"},

    ...
    },
  "eventstream":true
}

Motivation and Context

Testing

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed
• A short description of the change has been added to the CHANGELOG
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

[dagnir]

Note: anything that doesn't use the customization at the moment will fail to compile, e.g. the protocol-tests module since the model classes don't know how to call the new visitor methods yet (will be done in following PR). Right now they're hardcoded to call visitor.visit(this) basically.