Support for compressed and X.509 public key format

[skmcgrail]

Description of changes:

• Support for serializing ECDSA and ECDH keys in compressed and uncompressed EC point octet format, and support for X.509 SubjectPublicKeyInfo format.
• Support for parsing ECDSA and ECDH keys that may be in compressed, uncompressed, or X.509 format. First attempting to parse as X.509, and then falling back to EC point format before failing the parse attempt.
• X2559 supports X.509 as well, will follow up Ed2559 in a separate PR since that requires a bit more refactor work and wanted to simplify this PR a bit.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.47368% with 30 lines in your changes missing coverage. Please review.

Project coverage is 92.56%. Comparing base (c358484) to head (f9af638). Report is 61 commits behind head on main.

Files	Patch %	Lines
aws-lc-rs/src/ec.rs	88.07%	5 Missing and 8 partials :warning:
aws-lc-rs/src/agreement.rs	91.42%	7 Missing and 5 partials :warning:
aws-lc-rs/src/agreement/ephemeral.rs	88.57%	4 Missing :warning:
aws-lc-rs/src/ec/key_pair.rs	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #498 +/- ## ========================================== - Coverage 95.80% 92.56% -3.24% ========================================== Files 61 67 +6 Lines 8143 9136 +993 Branches 0 9136 +9136 ========================================== + Hits 7801 8457 +656 - Misses 342 417 +75 - Partials 0 262 +262 ```

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[ctz]

I think ideally this would have been done with a separate API (eg, agreement::ECDH_P256_COMPRESSED, agreement:ECDH_P256_AUTODETECT, perhaps). As it is, this is not compatible with:

• TLS1.3 (which outlaws all encodings other than uncompressed)
• TLS1.2 (which separately negotiates the encoding, then must only accept the negotiated encoding.)
• X.509 (which allows uncompressed and compressed and expects them to be self-describing, but obviously never should accept a SPKI recursively inside another id-ecPublicKey SPKI)