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

[vhartikainen]

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:

   firebase-ios-sdk/FirebaseAppCheck/Sources/DebugProvider/FIRAppCheckDebugProviderFactory.m

   Lines 29 to 34 in 8fe1ddc

   	// Print only locally generated token to avoid a valid token leak on CI.
   	FIRLogWarning(kFIRLoggerAppCheck, kFIRLoggerAppCheckMessageCodeDebugToken,
   	@"Firebase App Check debug token: '%@'.", [provider localDebugToken]);
   	return provider;
   	}

API Proposal

No response

Firebase Product(s)

App Check

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