feat: move client-side config headers to client-sdk (#211)

This commit moves the client-side `LDClientConfig` and
`LDClientConfigBuilder` out of `common` and into `client-sdk`.

Although the header locations are moving, no symbol name or function/struct definition is changing.

To ease this transition, stub headers in the original
locations log messages using `#pragma message`. These stubs can't
automatically include the new headers  because that'd introduce
a circular dependency.

Author: cwaldren-ld

examples/hello-c-client/main.c

@@ -1,7 +1,7 @@
 #include <launchdarkly/client_side/bindings/c/sdk.h>
 
-#include <launchdarkly/bindings/c/config/builder.h>
 #include <launchdarkly/bindings/c/context_builder.h>
+#include <launchdarkly/client_side/bindings/c/config/builder.h>
 
 #include <stdbool.h>
 #include <stdio.h>