[Resources.AWS] Move string literals to AWS Semantic Conventions class

[ppittle]

Changes

Move string literals to be constants in the AWS Semantic Conventions class. This is in preparation of a larger overhaul in how the AWS libraries will handle Semantic Conventions.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.16%. Comparing base (71655ce) to head (eae79ce).
Report is 623 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2363      +/-   ##
==========================================
+ Coverage   73.91%   79.16%   +5.25%     
==========================================
  Files         267       12     -255     
  Lines        9615      288    -9327     
==========================================
- Hits         7107      228    -6879     
+ Misses       2508       60    -2448     

Flag	Coverage Δ
unittests-Resources.AWS	79.16% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Resources.AWS/AWSEBSDetector.cs	92.59% <100.00%> (ø)
src/OpenTelemetry.Resources.AWS/AWSEC2Detector.cs	74.28% <100.00%> (ø)
src/OpenTelemetry.Resources.AWS/AWSECSDetector.cs	87.87% <100.00%> (ø)
src/OpenTelemetry.Resources.AWS/AWSEKSDetector.cs	64.28% <100.00%> (ø)

... and 271 files with indirect coverage changes