auth: have quota project override in credentials package

[codyoss]

We should allow users to explicitly set the value returned by quota project getter. This would also allow us to clean up so logic in the transport packages that is needed today because we can't pass the override from the client options down to the credentials package today. There is a TODO in #10807 about this work.

[codyoss]

Said logic:

google-cloud-go/auth/httptransport/transport.go

Lines 71 to 75 in d8a53d6

	qp, err := creds.QuotaProjectID(context.Background())
	if err != nil {
	return nil, err
	}
	if qp != "" {

and

google-cloud-go/auth/grpctransport/grpctransport.go

Lines 268 to 272 in d8a53d6

	qp, err := creds.QuotaProjectID(ctx)
	if err != nil {
	return nil, err
	}
	if qp != "" {