Add hook_myrest_blueprint_alter

Problem/Motivation

A site needs a point before generation where it can say "not this bundle", "also this endpoint", "this path instead". Without it, excluding a single bundle from the API means editing generated files, which the whole design forbids.

Proposed resolution

• Invoke hook_myrest_blueprint_alter() on the finished Blueprint, before anything is written.
• Document it in myrest.api.php with a working example that removes a bundle and one that renames a key.
• Validate the altered Blueprint again, so a hook cannot produce configuration the generator would choke on.
• Kernel test with a test module implementing the hook.

Remaining tasks

Everything.