Refactor ECDSA structs into ec::signature

[justsmth]

Description of changes:

• Migrate ECDSA structs/functions into ec::signature module.

Testing:

No changes to API detected.

❯ cargo public-api diff latest
...
Removed items from the public API
=================================
(none)

Changed items in the public API
===============================
(none)

Added items to the public API
=============================
(none)

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 81.00559% with 34 lines in your changes missing coverage. Please review.

Project coverage is 92.89%. Comparing base (c358484) to head (ae056f4).
Report is 146 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/ec/signature.rs	81.46%	20 Missing and 13 partials ⚠️
aws-lc-rs/src/ec/key_pair.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #654      +/-   ##
==========================================
- Coverage   95.80%   92.89%   -2.91%     
==========================================
  Files          61       68       +7     
  Lines        8143     9718    +1575     
  Branches        0     9718    +9718     
==========================================
+ Hits         7801     9028    +1227     
- Misses        342      402      +60     
- Partials        0      288     +288     

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