feat(auth): add signInWithCustomToken method

[dehypnosis]

Hi Team,

This patch adds a public method to call an API:

• https://cloud.google.com/identity-platform/docs/reference/rest/v1/accounts/signInWithCustomToken
• which exchanges custom token to Id token and refresh token.

Sorry if this PR is not appropriate in contribution process.

Background

Actually, I was using this REST API by calling that with API Key via separated HTTP client as the document described.

But recently, I had to turn Firebase AppCheck for Authentication due to SMS fraud issue. With AppCheck enabled, this API stops to work with "Invalid App Check Token" error in my server. I found no way to embed AppCheck header in internal call. So I forked and added this API call method to firebase admin to make it work with service account credentials. And it resolves the issue.

Please check this issue. Thanks.

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