[eas-cli] use existing google service account key

[quinlanj]

Checklist

• I've added an entry to CHANGELOG.md if necessary.
• I've tagged the changelog entry with [EAS BUILD API] if it's a part of a breaking change to EAS Build API (only possible when updating @expo/eas-build-job package).

Why

This PR:

• allows the user to assign an existing GoogleServiceAccountKey to their project on the www servers
• adds an action to assign an existing key in the credentials management workflow

It will also eventually be used in the submissions workflow.

Test Plan

• new tests pass
• manually tested new credentials management action

[github-actions]

Size Change: +2.17 kB (0%)

Total Size: 27 MB

Filename	Size	Change
./packages/eas-cli/dist/eas-linux-x64.tar.gz	27 MB	+2.17 kB (0%)

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