Feature/OIDC auth

[lukerosser]

Adds OIDC login support

OIDC metadata discovery is supported or URLs can be specifically provided through .env

Some potential discussion areas:

• Removes username and password fields from the login replaced by a single OIDC login button
• Automatically provisions a user following a successful OIDC login
• evtx2splunk has been rebuilt manually with a python-dotenv version bump to the next minor version, 0.21, to solve version conflicts with oic
• The other "oidc_proxy" auth method has been left untouched as it looks to support a different use case without direct integration by authenticating elsewhere and reading the resulting headers

[e-renna]

OIDC implementation (technically) fulfills #214 since MFA can be delegated to the auth provider

[whikernel]

Thanks a lot @lukerosser ! 🙏

@e-renna - I'll close #214 as MFA is already available since a few version 👍

[whikernel]

@lukerosser - I ported your changes to the current next release because I want this to be published asap and the develop branch may be delayed a bit.

For your - valid - points:

1. It's a valid points. For my test I directly redirects the user to the SSO portal if OIDC is set up. In a second time we'll see how we can handle the hybrid mode where users can be from OIDC and local for instance.
2. I added a condition on the already existing IRIS_AUTHENTICATION_CREATE_USER_IF_NOT_EXIST variable which allows to change this behavior.
3. Thanks, this module (which is mine) is a bit of pain and probably noone is using it, so it might disappear sometimes
4. Indeed, it's legacy - we'll probably remove at some point

Thanks again for the work 🙏