Migrate off of archived firebase/OCMock SPM dependency

[redfinwyland]

Is your feature request related to a problem you're having? Please describe.

The following dependency in the Package.swift has been archived for over a year by the firebase org:

.package(
      name: "OCMock",
      url: "https://github.com/firebase/ocmock.git",
      .revision("7291762d3551c5c7e31c49cce40a0e391a52e889")),

Describe the solution you'd like

The firebase author forked this from the original repo which appears to fully support SPM now:
https://github.com/erikdoe/ocmock/blob/master/Package.swift

What work would be needed to make this dependency change in this Package to ease dependency resolution in projects that depend on GoogleSignIn?

[mdmathias]

Hi @redfinwyland and thanks for the issue. Good callout that this dependency is archived. We'd happily accept a PR updating it. We will likely get to it eventually, but it's not at the top of our stack, so to speak.

[redfinwyland]

@mdmathias Can you please review the attached PR, thanks