Distributed event notification of GWC config changes

[groldan]

• Add missing gs-gwc.xml initialization for pgconfig

• Configure GWC lock provider with the cluster-aware geoserver lock provider

• Distributed event notification of GWC config changes

  Send a distributed event upon GWC config change and react accordingly
  on the receiving end by forcing a reload of the GWCConfig and applying
  the change to the ConfigurableBlobStore, as done in
  GWCSettingsPage.save().

---

Fixes #411

[groldan]

The backport to release/1.8.x failed:

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

stderr

hint: Recursive merging with submodules currently only supports trivial cases.
hint: Please manually handle the merging of each conflicted submodule.
hint: This can be accomplished with the following steps:
hint:  - come back to superproject and run:
hint:
hint:       git add config
hint:
hint:    to record the above merge or update
hint:  - resolve any other conflicts in the superproject
hint:  - commit the resulting index in the superproject
hint:
hint: Disable this message with "git config advice.submoduleMergeConflict false"
error: could not apply bf355168... Add missing gs-gwc.xml initialization for pgconfig
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Failed to merge submodule config (not checked out)
CONFLICT (submodule): Merge conflict in config

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-release/1.8.x release/1.8.x
# Navigate to the new working tree
cd .worktrees/backport-release/1.8.x
# Create a new branch
git switch --create backport-549-to-release/1.8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick bf3551686667d63560346f520bd019060fe38576,cc9065d9faef2bffd9d36221a86de5491276e3b4,695600906b4fca9e988ec84ccdb15ee7aca0360d
# Push it to GitHub
git push --set-upstream origin backport-549-to-release/1.8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/1.8.x

Then, create a pull request where the base branch is release/1.8.x and the compare/head branch is backport-549-to-release/1.8.x.