feat: generate three firebase discovery doc based APIs

[devoncarew]

Generate three firebase discovery doc based APIs:

• add definitions for fcm.v1.json (Firebase Cloud Messaging), firebaseappcheck.v1.json (firebase AppCheck), and firebaserules.v1.json (firebase rules)
• regenerate the packages

This is to exercise discovery doc support in sidekick. Note that I have local changes to sidekick (a bug fix, updates to the dart generation templates, ...) to generate what's in this PR. We'll need to upstream fixes and bug reports to sidekick before something like this can be checked in.

[devoncarew]

It looks like:

• we need to be able to handle 'any' types that don't specify a specific scalar type (currently only 'google.protobuf.Value' is supported); https://github.com/googleapis/discovery-artifact-manager/blob/master/discoveries/firebaserules.v1.json#L1038
• handle level 2 expressions in uri templates ('{+parent}/messages')
• handle trailing :foobar text in uri templates ('{+parent}/messages:send', 'v1/{+app}:exchangeAppAttestAssertion'); https://github.com/googleapis/discovery-artifact-manager/blob/master/discoveries/firebaseappcheck.v1.json#L160C20-L160C56, https://cloud.google.com/docs/discovery/build-client-library#compose-a-request
• for the Dart backend, title case the service names (we want title case for our classes; proto based APIs all use titlecase, but some discovery APIs - like FCM - use 'messages' as the service name)
• for the Dart backend, ignore some synthetic messages. these are used for namespacing by some backends, but not necessary for the Dart backend, and creates a name conflict with the associated service. So, we want the child request and response synthetic messages, but not the containing parent synthetic message

I'll create separate issues + PRs for the above.

[devoncarew]

I haven't tried using these APIs, but I'm now able to generate all three. The patches to sidekick may not all be landable as-is, but I'll put them somewhere (other than my local disk) for visibility.

sidekick.patch