Snapcraft-review-tools and store upload reviews don't like new fields in snap.yaml

ref: Need help of a reviewer :)

Adding any of the new metadata fields such as issues, source-code and donations cause any uploads of the built package to be held for manual review. This is because snapcraft-review-tools does not know about these fields.

An example snap file that has been built with these fields that is failing store review, if you want to have a poke to see it, is at https://public.apps.ubuntu.com/download/vaWdbpmiYyv4qgsmTaqGfVgJE5F592w6_136.snap?token=vaWdbpmiYyv4qgsmTaqGfVgJE5F592w6_136.snap.YW2Wag.AOoqCYtuvxZzMEgVD9spJTVmN0I

(If that URL is dangerous to expose, then please censor it. Then fix the service to allow public downloads of snaps uploaded to the public store because it is stupid to have it behind an authorisation mechanism)

The following in snapcraft.yaml:

issues: https://github.com/diddlesnaps/fakecam/issues
source-code: https://github.com/diddlesnaps/fakecam.git
donation: https://liberapay.com/diddledani

Will produce a meta/snap.yaml file with:

links:
  donation:
  - https://liberapay.com/diddledani
  issues:
  - https://github.com/diddlesnaps/fakecam/issues
  source-code:
  - https://github.com/diddlesnaps/fakecam.git

The review then complains about the links block with the following message:

unknown entries in snap.yaml: 'links' lint-snap-v2_unknown_field 
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