Preallocate correct-sized buffer for event stream (#4212)

## Motivation and Context

Currently when serializing event streams, an empty buffer is used. This
results in repeated reallocations as the buffer grows. However, we can
actually precalculate the exact size that is required.


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## Description
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This PR does two things:
1. Add a MessageSizeHint that aws-smithy-http can use to determine the
in-memory serialized size of the message
2. Add a benchmark of write_message. The benchmark includes an prototype
optimized implementation that we can switch to at a later date.

## Testing
- Benchmark
- Test of correctness for size calculation

## Checklist
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rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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