feat: add OAuth2 refresh token credential support

[demolaf]

Adds explicit support for OAuth2 refresh token credentials from a custom file path or raw parameters, for cases where the default ADC file location isn't sufficient.

• Adds Credential.fromRefreshToken(File) factory — reads and parses an OAuth2 refresh token JSON file (e.g. ~/my-project/credentials.json)
• Adds Credential.fromRefreshTokenParams({clientId, clientSecret, refreshToken, type}) factory for constructing the credential from individual parameters
• Adds unit tests covering valid credentials, missing/empty fields, malformed JSON, and unreadable files
• Adds integration test in firebase_app_prod_test.dart exercising the new credential against a real Firebase project

[gemini-code-assist]

Code Review

This pull request introduces support for OAuth2 refresh token credentials in the Firebase Admin SDK. It adds new factory methods, Credential.fromRefreshToken and Credential.fromRefreshTokenParams, along with the RefreshTokenCredential implementation which handles auto-refreshing authenticated HTTP clients. The changes also include comprehensive unit and integration tests, and updated documentation for running these tests. One review comment suggests improving the fromRefreshTokenParams API by making the type parameter optional with a default value, as it is currently required but not utilized in the internal constructor.