Add secret manager for storing and loading SA keys

[stephen-soltesz]

This change adds a new type adminx.SecretManager that uses the adminx.ServiceAccountManager to load or create and store service account keys in Google Cloud Secret manager.

With this change and the adminx package, it should be possible to add service account key distribution to the Register API so that new nodes can join using in-band configuration.

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 10689240323

Details

• 96 of 96 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 98.568%

---

Totals
Change from base Build 10687864932:	0.2%
Covered Lines:	895
Relevant Lines:	908

---

💛 - Coveralls

[nkinkade]

LGTM

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained

[stephen-soltesz]

Thank you!