CSS-6764 update cloud creds

[kian99]

Description

This PR builds on #1127, it adds a facade method for UpdateServiceAccountCredentials. The facade method calls the same an underlying JIMM method as the Juju facade handlers for updating user cloud credentials.

Note that the rpc handler checks that the user has permission to manage the service account, we do this check in the jujuapi package whereas we normally do this check in the jimm package. I've had to write the tests slightly differently from the norm to account for this - specifically I've add an exported SetUser function in JIMM's controllerRoot.

Fixes CSS-6764

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

[alesstimec]

LGTM modulo a few comments

[ale8k]

lgtm bar ales' comments

[alesstimec]

nitpick: doesn't need to be exported.. you can create a method in export_test.go

var SetUser = func(r *controllerRoot, u *openfga.User) {
	r.mu.Lock()
        r.user = u
	r.mu.Unlock()
}

[kian99]

Done.