Dispatch an event to adjust cacheable metadata after a facet is built

Since the caching of facets in combination with Search API and Views has been improved, we should offer some more features.
Right now the cacheable metadata is gathered from the view, the search api query and facets' processors.
This works quite well for the defaults, but there's no possibility to force a more aggressive caching.
For example the language, the query parameters or the user's rights are mostly set as cache context.
Especially in cases you display a facet without the rendered source it might be possible to cache the facet regardless of these contexts.
So we should provide a possibility to force specific metadata.