WIP: SAML support (backend)

[SimonTagne]

This is the back-end part for SAML authentication support.

Here are a few points where I would appreciate some advice:

• The configuration in json is not really readable, especially when including PEM certs/keys
• The constants for the authentication cookies are defined in the client source tree, and I don't know any good solution to access it in the ACS endpoint (it's hardcoded for now)

I'll start working on the front-end now and keep you updated

[meltyshev]

The configuration in json is not really readable, especially when including PEM certs/keys

For the first version, I think it's fine. At some point we'll need to switch to a different format of settings 🤔

The constants for the authentication cookies are defined in the client source tree, and I don't know any good solution to access it in the ACS endpoint (it's hardcoded for now)

I’m also having this problem that there is no shared place with constants. I would just add a todo note, so as not to forget to take them out later when there is a shared place.

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