Reduce memory allocations for non-streaming async requests

[alextwoods]

Reduced memory allocation for non-streaming requests made with async clients by reading the marshalled request body into a single correctly-sized buffer, instead of copying it through a growing intermediate buffer on every attempt

Motivation and Context

Profiling identified that for non-streaming bodies in the async client, we were unessecarily copy/allocating more than a full copy (because of the growing intermeidate buffer) of the request body in the SimpleHttpContentPublisher. We generally have content-length available and can use that to right-size a buffer. This change doesn't avoid the full copy (that would require larger interface updates), but at least it avoids allocating a growing intermediate buffer.

Note this is an async client only problem, sync doesn't use this path.

Modifications

Testing

Added new unit tests to cover the full SimpleHttpContentPublisher.

Benchmarks

baseline	scoped-down
tiny — time	22.898 ± 1.829 µs/op	19.221 ± 0.517 µs/op (−16.1%)
tiny — alloc	52,829 ± 322 B/op	48,776 ± 97 B/op (−7.7%)
350 KB — time	468.343 ± 21.902 µs/op	409.561 ± 13.794 µs/op (−12.5%)
350 KB — alloc	2,195,108 ± 2,071 B/op	1,145,135 ± 334 B/op (−47.8%)

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 added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.

License

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

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