Add cmac sign_secret

[Elvrarin]

Description of changes:

Adds SecretTag and sign_secret.

Testing:

cargo test, make clippy, make format, all passes

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

❌ Patch coverage is 33.33333% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.39%. Comparing base (c358484) to head (b959931).
⚠️ Report is 293 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/cmac.rs	33.33%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #960      +/-   ##
==========================================
- Coverage   95.80%   92.39%   -3.41%     
==========================================
  Files          61       74      +13     
  Lines        8143     9915    +1772     
  Branches        0     9915    +9915     
==========================================
+ Hits         7801     9161    +1360     
- Misses        342      473     +131     
- Partials        0      281     +281     

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