Fallback to current class if loading a Jsonp provider using ServiceLoader fails

[swallez]

Fixes #163

Calling JsonProvider.provider() invokes ServiceLoader that tries to find an implementation from the current thread's context classloader. In some environments where the context classloader isn't correctly set, this can cause a failure to find an implementation, even if the deployment was otherwise correct.

This PR enhances the provider lookup when the standard method fails by first trying to find an implementation in the Java client's classloader, and else in the JsonP SPI library's classloader.

It also introduces the forbidden-apis build plugin to make sure we will not inadvertently reintroduce calls to JsonProvider.provider().

[github-actions]

The backport to 8.1 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-8.1 8.1
# Navigate to the new working tree
cd .worktrees/backport-8.1
# Create a new branch
git switch --create backport-173-to-8.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 8389a694c022cb91de2307f65fe8a79d7e843c1e
# Push it to GitHub
git push --set-upstream origin backport-173-to-8.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.1

Then, create a pull request where the base branch is 8.1 and the compare/head branch is backport-173-to-8.1.