New endpoints for l10n_client_contributor

Thanks for your contribution @Harlor! While I've never dealt with REST endpoints myself, I added some minor comments, some of them less minor, such as permission issues.

Once we are done with this, ideally you should enable the module on the platform on gitlab.com and open an MR there for core.extension.yml.

Just so I can get a general idea for the Localize project, do you have an idea of how much work is left, and what time you might be able to dedicate in a month? In your opinion, how many developers should be working on this?

(P. S.: I still have your charger, I'll need your email address to finalize shipping.)

• Remove Unused JSON:API Resource Configurations and Adjust Dependencies
• Deleted obsolete JSON:API resource configuration files:
• jsonapi_extras.jsonapi_resource_config.l10n_server_string.yml
• jsonapi_extras.jsonapi_resource_config.l10n_server_translation.yml
• jsonapi_extras.settings.yml
• Removed unnecessary dependency on jsonapi_extras module.

Thanks! Next steps:

• Figuring out why we need the L10nRemoteSubscriber event subscriber for, and either remove debug traces or remove it completely.
• Properly check access in L10nServerContributorResource::access(): user should be a member of the translation team. That being said, we most certainly should implement access checks properly in #3437137: Translation suggestion creation and moderation don't provide group permission instead.
• Deal with authentication (#3395488: Localize client authentication with d.o infrastructure), instead of the temporary basic auth mechanism.

Note the proposal I made in https://www.drupal.org/project/l10n_client/issues/3395488#comment-16065118