Support Basic authentication in SCIM provider

**Is your feature request related to a problem? Please describe.**
I'm trying to set up SCIM for Unity (which is in beta on their side, so details might change). They only support Basic authentication as the method to authenticate to the SCIM service.

**Describe the solution you'd like**
Ability to choose between Bearer and Basic authentication when creating a SCIM provider. Basic authentication could either ask for two fields (username and password), or only one (which usually is username:password base64 encoded).

**Describe alternatives you've considered**
Afaik there are no alternatives at the moment.

**Additional context**
None.

Locate the SCIM provider creation flow and the existing Bearer authentication handling, then trace how credentials are used for SCIM requests. Done means a provider can select Basic or Bearer authentication and Basic authentication works with the requested credential fields.

Written by the indexing model from the issue text.