Add subplat-cirrus to repositories.yaml

[relud]

fixes EXP-6110

[sean-rose]

question (non-blocking): Based on the dependencies including glean-server, is this perhaps intended to be specific to the SubPlat backend? Or would it also be used for the SubPlat frontend?

If it is specific to the SubPlat backend, then I think it'd be good to change the app_name, v1_name, and app_id to be more similar to the values for the existing subscription_platform_backend app (plus the appropriate Cirrus-specific suffix).

[relud]

tl;dr this isn't backend specific

More precisely, while we expect only the backend to use it, we expect any experiments in the frontend to receive feature configuration from the backend, not directly from cirrus. This matches with accounts_cirrus which is also only used in the backend.

integrating with cirrus on the frontend would be for applications that don't have a suitable backend, e.g. static sites that only have client-side code.

[relud]

thanks for the reviews, i will need someone else to merge this as I lack such powers.