[FR]: Add logging to indicate if debugtoken passed via environment variable FIRAAppCheckDebugToken is used

### Description

1. When configuring AppCheck to use Debug Token from Environment variable `FIRAAppCheckDebugToken`, indicate that this is in use (`currentDebugToken`), instead of the generated Debug Token (`localDebugToken`)
2. Currently Firebase SDK outputs the `localDebugToken`, but doesn't indicate whether it is in use or the one passed via environment variable.
3. Add logging statement to indicate that `currentDebugToken` is in use. DONT output the token, as that can be seen as security issue, as also commented in the code. See: https://github.com/firebase/firebase-ios-sdk/blob/8fe1ddc5f16957da5b2dc8b1dee90e5bd85205ff/FirebaseAppCheck/Sources/DebugProvider/FIRAppCheckDebugProviderFactory.m#L29-L34

### API Proposal

_No response_

### Firebase Product(s)

App Check