Add webdriver test for SAML and mock IDP tool

[bduffany]

This PR is a precursor for rolling forward https://github.com/buildbuddy-io/buildbuddy/pull/6077 and should also give us more confidence making auth code changes going forward.

It does the following:

• Adds a local mocksaml IDP server binary which can be used to run an IDP locally. It currently uses https://github.com/boxyhq/mock-saml running inside a local docker container. The container is pretty lightweight, but in the future we could potentially swap this out for something even lighter (e.g. a tiny HTTP server we write ourselves).
• Adds a webdriver test which runs this local mocksaml server and links the IDP to a test org, then logs in via the SAML IDP webpage.
• Adds a README with instructions on running the tool and developing SAML locally.

Related issues: N/A