feat: interface for custom login webflow

Description

Added an interface for insertion of custom login flow states and actions.

How to test

Add new config entries to cas.yaml with a structure like this:

perun:
  filters:
    - filter-name: "perunUserShowcase"
      config:
        key1: "value1"
        key2: "value2"
    - filter-name: "filter2"
      config:
        key3: "value10"
        key4: "value20"
cas:

As you can see, the new stuff isn't part of the cas scope but of a newly added perun scope. firstFilterName is the name of the first filter. (There has to exist a state with that name which is named filterName + State. so here it would be perunUserState)

Build CAS with overlay in cas-sites. Add this logger to log4j2.j2: <Logger name="org.apereo.cas.web.flow.perun" level="DEBUG" /> (This is probably only required if web flow logger isn't already set but I recommend to remove almost all other loggers for this test.)

Run CAS and look at the logs.

Author's checklist

• I have followed the contribution guidelines
• This MR has been tested or does not change functionality
• I have added relevant merge request dependencies (if this MR has any)
• I have added the correct labels
• I have assigned reviewers (if any are relevant)
• [not yet] I have edited the documentation (if the changes require it) or I have noted the need for the change if I do not have access to the documentation
• I have marked all introduced BREAKING CHANGES or necessary DEPLOYMENT NOTES in the commit message(s)

Reviewer's checklist

• This MR has been tested or does not change functionality
• This MR has correct commit message format

Other information

There will also be a guide how to use this interface. Will be linked in the task description.

Related issues

Closes STR-1494