[FR] App Check custom token support

**Is your feature request related to a problem? Please describe.**
For now we have to use NodeJS in order to use App Check tooling for our non-commodity android business hardware setup. We would prefer using a Kotlin/Java since it fits in our android + spring boot eco system. We are currently wrapping NodeJS Code in a cloud function which is called by our backend service which already uses the firebase admin sdk for java. So it is a really unnecassry piece of infrastructure we want to get rid of as soon as possible.

**Describe the solution you'd like**
So far only the NodeJS Admin SDK supports to implement a custom token provider. We would appreciate that this functionality is present also in the java admin sdk

Looking forward to use custom token provisioning in java. Thank you