[ELY-2807] Allow complete provider url in the oidc config

[MarcoBau]

https://issues.redhat.com/browse/ELY-2807

[fjuma]

Please update the base branch for this PR to 2.x.

If you could also add a test for this, that would be great.

Thanks!

[MarcoBau]

Hi,
we need that change in the 1.19.x branch.
I can open another MR to the 2.x branch if you want.

I'm checking how I could write a meaningful test around that change

[MarcoBau]

The method getDiscoveryUrl() where the change is done is private and its result is never exposed in a way a test could be written to check its result.
The only way I see to test it is to relax the method's visibility making it package protected. Should I proceed with a change like that or do you have another suggestion?