Allow to retrieve add-ons by author IDs when using the Search API endpoint

[willdurand]

The Search API has a query parameter named author that allows to retrieve add-ons given a comma-separated list of author username: https://addons-server.readthedocs.io/en/latest/topics/api/addons.html#search.

An issue reported on addons-frontend highlighted the fact that usernames changed (sometimes) and that using username was not always ideal.

It would be more reliable to pass a list of author IDs, so can we have either a new query parameter OR allow numeric IDs in the existing author parameter?

[diox]

We have the ids already in ES, but not currently indexed, so it would require a full reindex. Code-wise it'll probably look like what we do for AddonExcludeAddonsQueryParam which similarly supports both add-on slugs and ids.

[willdurand]

This will be useful for #5528 too.

[AlexandraMoga]

Verified fixed on dev:
User IDs can be passed for the author search parameter in order to retrieve add-ons (all types) submitted by the requested users.

see https://addons-dev.allizom.org/api/v4/addons/search/?author=10638484,6107817,10641570,4230

Mixing user IDs and slugs will also retrieve the correct results,