add Sigv4A support (#2939)

## Motivation

smithy-rs#1797

## Description

This PR adds Sigv4a support along with many tests that were ported from
the CRT Sigv4a project. **It does not include**:
- An E2E test. We should create one, but doing so requires us to set up
MRAP which is a good amount of work.
- A fuzz test. While I wrote and ran one on my machine, I found the fuzz
testing tool to be really difficult to use and it would take some work
to include fuzz testing in CI.

Additionally, test Sigv4a is annoying because Sigv4a signing is
non-deterministic. Because of this, the tests test signature generation
by first signing a request, and then by verifying that signature (_This
is how the CRT tests this feature_).

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Co-authored-by: ysaito1001 <awsaito@amazon.com>