Add tokenlist api

We need an api to call via ajax to determine the available tokens for a given entity. An example is the Scheduled reminders UI which currently presents the tokens for all entities (available or not) but would ideally look up the tokens via ajax once the entity is selected.

I think the hardest part of doing this is agreeing how it should look - eg. entity, action, parameters - eg

Civi\Api4\Tokens::list()
  ->setSchema(['contributionId', 'contactId')
   ->setWhere('contact_type', '=', 'Organization')
  ->execute();

Possible entity names

• Token
• Tokens
• TokenProcessor
• EntityTokens

Possible actions

-list

• getTokens
• listTokens

Schema - maybe this one is clear cut

And - how do we pass relatively free form tokens such as activity_type_id, contact_type or (possibly in the future) saved_search_id where search kit plays a role