This PR partially addresses #1456, providing SSO SAML support. The flow of the code is as follows.
A Kolide user attempts to access a protected resource and is directed to log in.
If SSO identity providers (IDP) have been configured by an admin, the user is presented with SSO log in.
The user selects SSO, which invokes a call the InitiateSSO passing the URL of the protected resource that the user was originally trying access. Kolide server loads the IDP metadata and caches it along with the URL. We then build an auth request URL for the IDP which is returned to the front end.
The IDP calls the server, invoking CallbackSSO with the auth response.
We extract the original request id from the response and use it to fetch the cached metadata and the URL. We check the signature of the response, and validate the timestamps. If everything passes we get the user id from the IDP response and use it to create a login session. We then build a page which executes some javascript that will write the token to web local storage, and redirect to the original URL.
I've created a test web page in tools/app/authtest.html that can be used to test and debug new IDP's which also illustrates how a front end would interact with the IDP and the server. This page can be loaded by starting Kolide with the environment variable KOLIDE_TEST_PAGE_PATH to the full path of the page and then accessed at https://localhost:8080/test
