Add staging flag to initialize with staging TUF metadata

[aaronlew02]

Closes #4533

Summary

This change adds a --staging flag to cosign initialize which initializes the TUF cache directory with staging TUF metadata.

This change also clears the TUF cache directory on any run of cosign initialize so that prod and staging TUF metadata do not coexist.

This functionality has been tested locally.

Documentation

sigstore/docs#404

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.34%. Comparing base (2ef6022) to head (f28ce15).
⚠️ Report is 603 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/cosign/cli/initialize/init.go	0.00%	4 Missing and 2 partials ⚠️
cmd/cosign/cli/initialize.go	50.00%	2 Missing ⚠️
cmd/cosign/cli/options/initialize.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4561      +/-   ##
==========================================
- Coverage   40.10%   36.34%   -3.76%     
==========================================
  Files         155      220      +65     
  Lines       10044    12301    +2257     
==========================================
+ Hits         4028     4471     +443     
- Misses       5530     7136    +1606     
- Partials      486      694     +208     

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