Use a local SAML IdP provider instead of Okta

[marcusdacoregio]

To avoid relying on an external server and their rate limits, we should setup a local SAML IdP with Docker and use Testcontainers to run the Docker image on integration tests.

https://hub.docker.com/r/kristophjunge/test-saml-idp/

[jzheaux]

I'm reopening so that we can backport this to earlier branches. Also, the Java Config and XML samples need to be updated.