Add support for `g3w-admin-mapproxy` plugin (tiles cache provider)

The backport to v3.9.x 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-v3.9.x v3.9.x
# Navigate to the new working tree
cd .worktrees/backport-v3.9.x
# Create a new branch
git switch --create backport-589-to-v3.9.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 3ddae8190380ad86802edb4a74db3af21820c5c0
# Push it to GitHub
git push --set-upstream origin backport-589-to-v3.9.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-v3.9.x

Then, create a pull request where the base branch is v3.9.x and the compare/head branch is backport-589-to-v3.9.x.

g3w-suite / g3w-client

Add support for `g3w-admin-mapproxy` plugin (tiles cache provider) #589