pull request files collection could be more efficient

[MoralCode]

We might be able to squeeze a lot more performance from our api calls in PR files collection

Sonnet 4.6 has identified that we are constructing one graphql call per pull request:

https://github.com/chaoss/CollectOSS/blob/96adf3a4d68725db21622673ee6613693c0f5ace/collectoss/tasks/github/pull_requests/files_model/core.py#L52-L69

Some of these may return very few files.

Other places in the code already batch multiple queries into one graphQL call:

https://github.com/chaoss/CollectOSS/blob/96adf3a4d68725db21622673ee6613693c0f5ace/collectoss/tasks/frontend.py#L207-L212

We should do the same in pull request files to help make things faster (less waiting for network calls)

[MoralCode]

this ones relatively easy since we already use graphql for the queries