Pattern Directory Explorer: Introduce new block pattern categories endpoint

[ntsekouras]

Introducing a new block pattern categories endpoint: '/wp/v2/pattern-directory/categories'. This endpoint is for remote requests to api.wordpress.org/patterns/1.0/?categories.

It's part of the Pattern Directory Explorer, an enhancement for browsing the pattern directory from the editor.

Related GB PR: ​https://github.com/WordPress/gutenberg/pull/45749

Trac ticket: https://core.trac.wordpress.org/ticket/57551

Addition of the new endpoint for Pattern Directory categories.
Related GB PR: ​https://github.com/WordPress/gutenberg/pull/45749

Since I'm not sure if the ​Patterns Directory integration will be merged in time for 6.2, let's wait a bit for this PR.

Even though I think this endpoint wouldn't hurt to land in core, it isn't actually used anywhere yet, so it might be better to land when the integration lands. --cc @hellofromtonya

As this is part of a larger initiative, if that initiative might not land in 6.2, then yes I agree this PR should not go in yet. Why? Its purpose is for the Pattern Directory Explorer. Committing it now without the explorer being committed means the endpoint isn't used in Core and there's a potential for BC concerns should the endpoint need to change as the explorer continues in development and testing.

[hellofromTonya]

Pausing the introduction of this endpoint until there's confirmation the Pattern Directory Explorer feature will also land in 6.2.

The connection with Pattern Directory will not be ready for 6.2, so I'm closing this PR.