refactor: extract Firestore emulator credentials to separate emulator package

[lqiu96]

Summary

Refactors the Firestore emulator credentials implementation in GcpFirestoreEmulatorAutoConfiguration:

• Moves the custom Credentials implementation from an anonymous inner class in com.google.cloud.spring.autoconfigure.firestore into com.google.cloud.spring.autoconfigure.emulator.FirestoreEmulatorCredentials.
• Removes the regex excludeFilters previously added to FirestoreRepositoriesAutoConfigureRegistrar.

Rationale

In Java 8 runtimes, Spring Data's repository component scanning inspects all classes in the auto-configuration package. An anonymous inner class extending com.google.auth.Credentials (which now carries JSpecify 1.0.0 annotations targeting Java 9 ElementType.MODULE) triggered an ArrayStoreException on Java 8. Moving this internal credentials class to a dedicated emulator package prevents Spring Data from scanning it as a repository candidate without requiring any regex filters or test package modifications.

Compatibility

Zero breaking impact. The previous class was a private anonymous inner class; its return type, public API, and header injection behavior (Authorization and google-cloud-resource-prefix) remain identical.