google/externalaccount: support memory CredentialSource for externalaccount package

[matheuscscp]

Fixes #772

This change introduces memory sourced credentials for the
google/externalaccount package. This is for credentials
that are already in memory and don't need to be loaded
from a file or URL.

I have tested this e2e and it works:

access_token = ya29.c.c0ASRK0G...<redacted>...azdkII
email = [email protected]

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Patch Set 1:

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Message from Matheus Pimenta:

Patch Set 1:

(1 comment)

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Patch Set 2:

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Patch Set 3:

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