Ads: return `publisher`, `tags` and more in the endpoint

[humitos]

There are some data that comes from /sustainability/ (-ext) about the publisher, if it's ad free or not and more that should be considered at the moment the sponsorship addon is rendered at https://github.com/readthedocs/readthedocs-client/blob/main/src/sponsorship.js#L12

I think it makes sense to return all this data in the /readthedocs-config/ endpoint under addons.ethicalads key.

Reference API: https://github.com/readthedocs/readthedocs-ext/blob/4fe6331fac357a44f5e1be3879d7475ce560ffe6/readthedocsext/donate/restapi/views.py#L52