Add handling of Filtered Requests in LbAPI

This request adds the correct PPG approval check for Filtered Requests, as well as tagging a request with the appropriate label.

PPG Approval for Filtered Requests

Following !49 (merged), this request adds in the ability to correctly check for PPG approval in filtered requests

TODO:

• Modify way so it also deals with DIGI files and non 'full' requests (eg lhcb-simulation/mc-requests!123 (merged))

This is solved by assuming that every request is of the MDST limit, so it will assume so unless the request is specifically DST

Tagging a request with the appropriate label

Following the discussion in the sim meeting at 4 July 2023, it was suggested to add the 'Filtered Request' label for filtered request. The workflow is as follows:

1. If stage0 & filtering-script-like, add filtering tag
2. if stage > 0, just add ‘expert’

With the idea that if there are things above stage 0, then having the 'expert approval' tag should be sufficient.

@admorris @cburr