Convert apiquery to backfill

[piuccio]

Backfill is now an object with type and query. This allows future extensions to queries that are not backed by capi.

As a first step both apiQuery and backfill are available, the client tries to use backfill first and apiQuery if missing.

There's a change in a public method interface so I've created a new method with a different signature, users of the library have time to migrate.

@janua @DiegoVazquezNanini @davidfurey

[piuccio]

I've update this PR, it now has a BackfillResolver that implements two types of backfill query

• capi: the traditional capi search string
• collection: inherit the curated content of a parent collection

I don't think the child collection should inherit the parent's backfill, at least until we solve loops and deduping.

@janua I've added unit tests!

[piuccio]

thanks @janua I've fixed your comments.

I prefer to throw an exception when the backfill is invalid because it's a configuration error, it's something that shouldn't happen. Returning an empty list will silently ignore an invalid state. I've added a test for that

[piuccio]

@janua I've updated the PR with a better (???) backfill resolver

[janua]

Looks good :clap: :+1: