Please bring back IAMv1 support

**Is your feature request related to a problem? Please describe.**
A year and a half ago, [I filed an issue](https://github.com/googleapis/google-api-php-client-services/issues/2439) about missing properties from the `Google\Service\Iam` class related to service accounts. The somewhat unsatisfactory response was "v2 is now the dominant version, so support for v1 was dropped." This effectively prohibits anyone from updating to a version of 0.286.0 or newer, if they use the library to create service accounts.

**Describe the solution you'd like**
I would like to have support for **both** v1 and v2 of the IAM API. As we are nowhere close to feature parity, I am lead to believe this is not Google's intent and both versions have different purposes. This is further strengthened by the fact that both the Java client and the Go client have support for both versions:

* Java: https://github.com/googleapis/google-api-java-client-services/tree/e9f53d09c887872299a036d55db70363baade75a/clients/google-api-services-iam
* Go: https://pkg.go.dev/google.golang.org/api@v0.204.0/iam

**Describe alternatives you've considered**
A suggestion in the previous issue was using low level calls to interact with the API. This is not a viable option as we prefer readable and maintainable code for our projects.