Add 1Password as a backing store

[ccope]

Feature description

1Password has a plugin API, it would be nice if all of our developer secrets were stored in the same place.

[mjcheetham]

Hey! 👋 1Password support is something that's on our radar. We'll hopefully have more to share in the near future.

[hickford]

Have you tried https://github.com/develerik/git-credential-1password ? You can configure Git to use multiple credential helpers. Git queries each helper in turn for credentials. Successful credentials are stored in all helpers.

[credential]
	helper = 1password
	helper = manager
	credentialStore = cache
	cacheoptions = --timeout 0

The last two lines effectively disable GCM's built in credential storage.

[mjcheetham]

One issue you may face doing this is some of the more advanced handling of multiple accounts, such as is coming with #1190.

Here we need to be able to inspect all available credentials and accounts before making a determination about which to return. Storage of arbitrary metadata isn't in the helper protocol.

Sadly, the relying on Git's helper model isn't flexible enough to do these sorts of advanced scenarios. What about one-time-use or request-bound/signed credentials? Storing the actual credential doesn't make sense here, but instead we want to store something else like some generating key/credential used to create or sign the cred that gets used.