Add support for frame aliases

[WilliamJamieson]

This PR adds support for setting aliases for frame names. This allows alternate strings to be used to reference a given frame in the WCS.

Tasks

• Update or add relevant gwcs tests.
• Update relevant docstrings and / or docs/ page.
• Does this PR need a changelog entry? (If not, label with no-changelog-entry-needed.)
  • Write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types).

News fragment change types:

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.breaking.rst: any change or deprecation of the public API
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.83%. Comparing base (51d19be) to head (af676d5).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
gwcs/wcs/_pipeline.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #721      +/-   ##
==========================================
+ Coverage   85.78%   85.83%   +0.05%     
==========================================
  Files          41       41              
  Lines        4340     4364      +24     
==========================================
+ Hits         3723     3746      +23     
- Misses        617      618       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.