[#492] Refactor app warnings

[shadcn]

Fixes #492

This PR

1. Adds a warning for appe with all credentials revoked.
2. Moves the warning checks from the AppListBuilder into a separate service: \Drupal\apigee_edge\Entity\AppWarningsChecker so that it can be re-used (example in Apigee Kickstart).
3. Adds a Warnings extra field to app entities for showing warnings in entity view displays.

[shadcn]

I'm still working on this (see draft status). Not ready for review. Thanks @mxr576

[mxr576]

I'm still working on this (see draft status). Not ready for review. Thanks @mxr576

Yeah I know, I am just wanted to share my ideas to help to design this.

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[arlina-espinoza]

@googlebot I consent.

[arlina-espinoza]

@arshad I made some updates to the AppWarningsCheckerTest and AppListBuilderTest to make sure they also pass when integration is enabled during tests (ie, real API connection).

[arlina-espinoza]

LGTM 👍

[mxr576]

Raised a new issue as a follow up of a conversion in this PR ^