Generate an inlined new-type struct with an async .send() method for aws_smithy_http::event_stream::receiver::Sender

[ysaito1001]

Context: #3114 (comment)

Just as we generated an inlined new-type with an async recv() method for aws_smithy_http::event_stream::receiver::Receiver, the Sender can also benefit from having a similar new-type.

There is a question of what to new-type from the aws_smithy_http::event_stream::sender because there are two structs Sender and MessageStreamAdapter. Given that the former is merely used to create the later, it probably makes sense to new-type the latter, but that needs to be double-checked.

Since we currently do not expose Sender (or MessageStreamAdapter) in public API within stable crates, this work can be done with a backwards compatible manner.