crd: add runtimeFlags

[wasaga]

Summary

Adds runtimeFlags section to the CRD, so that certain experimental or subtle behaviour changes may be altered.

Related issues

Fixes: https://github.com/pomerium/ingress-controller/issues/1005

Checklist

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

[backport-actions-token[bot]]

The backport to 0-26-0 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-0-26-0 0-26-0
# Navigate to the new working tree
cd .worktrees/backport-0-26-0
# Create a new branch
git switch --create backport-1010-to-0-26-0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 d61398b0c7a20ce529f6f3f605fb1817d6f20e2f
# Push it to GitHub
git push --set-upstream origin backport-1010-to-0-26-0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-0-26-0

Then, create a pull request where the base branch is 0-26-0 and the compare/head branch is backport-1010-to-0-26-0.