feat: Add Credential Information to GoogleCredential classes

[lqiu96]

See b/360939866 for more info.

Returns a mapping of how the Credential classes were initialized:

• Source: Where the initialization information came from (Well known file, Env var)
• Name: User-friendly Credential class name
• Principal: Credential identity

[zhumin8]

LGTM. Just a few nit comments.
A question on UserCredentials: the design says to use account from credential json if present. I don't see relevant logic in user cred. What is the reason for this?

[lqiu96]

LGTM. Just a few nit comments. A question on UserCredentials: the design says to use account from credential json if present. I don't see relevant logic in user cred. What is the reason for this?

I missed that part. The account logic should be added now.

[zhumin8]

A few nits, non-blocking

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)

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[zhumin8]

LGTM