What does this implement/fix? Explain your changes.

This PR refactors the way AbstractConnectionResolver handles DataLoaders. More specifically:

• Child classes now define their loader names in ::loader_name()
• ::get_loader_name() is no longer an abstract method, and instead calls ::loader_name() if it has not already been set as a class property.
• Added a new graphql_connection_loader_name filter to more easily apply custom data loaders to an existing Connection Resolver (e.g. CPT-specific).
• The AbstractConnectionResolver::$loader property now only gets instantiated once, no matter how many times ::get_loader() is called. As a result, all direct calls to ::$loader have been replaced with ::get_loader().
• The overloaded ::get_loader_name() methods in the child classes have been replaced with ::loader_name().

There are no breaking changes in this PR.

Does this close any currently open issues?

Part of #2749

Any relevant logs, error output, GraphiQL screenshots, etc?

Any other comments?

• This PR is based on refactor: deprecate camelCase methods in AbstractConnectionResolver for snake_case equivalents #3083 , which should be merged first.
• Due to the way these changes are implemented, this PR is fully backwards compatible. Developers will be able to update their classes at their leisure to "opt in" to the new features like no instantiation, global filters, etc.

Where has this been tested?

Operating System: Ubuntu 20.04 (wsl2 + devilbox + 8.1.15)

WordPress Version: 6.4.3