This adds a new Nimbus component that will act as a wrapper around the uniffi generated Kotlin code from mozilla/nimbus-sdk, as well as be a point of Glean integration, at least initially.

- Integrate service-nimbus with samples-glean for testing
- Set up default endpoint for debug and release, debug pointing at the dev Kinto endpoint and release at the production endpoint.
- Adds the documentation on how to set up the Kinto dev endpoint.
- Updates samples-glean README with Nimbus wording