authenticate: add CORS headers to jwks endpoint

[calebdoxsey]

Summary

To give any javascript application the ability to query this endpoint, we should add CORS headers.

Related issues

• describe the special jwt endpoint /.pomerium/jwt  documentation#101

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[coveralls]

Coverage decreased (-0.05%) to 66.35% when pulling 63191f6 on cdoxsey/jwks-cors into e9e52d8 on main.

[backport-actions-token]

The backport to 0-18-0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-0-18-0 0-18-0
# Navigate to the new working tree
cd .worktrees/backport-0-18-0
# Create a new branch
git switch --create backport-3574-to-0-18-0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 5f51510e91d3282b386d5893ed4168bf793d8c6f
# Push it to GitHub
git push --set-upstream origin backport-3574-to-0-18-0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-0-18-0

Then, create a pull request where the base branch is 0-18-0 and the compare/head branch is backport-3574-to-0-18-0.