OpenID Configuration

**Describe the bug**
There are several issues with the current OpenID Implementation. I'm using Keycloak v17.0.2 as OIDC provider:

1. The values read from `application.properties` do not match the values described in the documentation.
These are the actual values read from `application.properties`:
https://github.com/alfio-event/alf.io/blob/2.0-M4-2204/src/main/java/alfio/manager/openid/OpenIdConfiguration.java#L81-L93
And these are the values described in the documentation:
https://github.com/alfio-event/alf.io/blob/2.0-M4-2204/website/content/en/docs/Configuration/OpenID/_index.md

2. The docs don't mention that you need to enable the `openid` Spring profile.

3. The docs mention that organization fields should be specified as `organization1/SPONSOR`, however the parsing code actually expects 3 parts, separated by `/`:
https://github.com/alfio-event/alf.io/blob/2.0-M4-2204/src/main/java/alfio/manager/openid/AdminOpenIdAuthenticationManager.java#L147-L148
A workaround is to instead specify the organization as `/organization1/SPONSOR` but I don't think this is intended.

4. Alf.io expects a client scope per claim specified in the `.properties` file. This is usually not how scopes and claims are used. Relevant code:
https://github.com/alfio-event/alf.io/blob/2.0-M4-2204/src/main/java/alfio/manager/openid/AdminOpenIdAuthenticationManager.java#L122-L128
Instead, users should be able to specify requested scopes in the `application.properties` file like `openid.scopes=openid,email,myprofile`. For example, check out how Grafana implements Generic OAuth: https://grafana.com/docs/grafana/latest/auth/generic-oauth/
The workaround is to create a dummy client scope per used claim in Keycloak.

5. I would love to be able to configure the used claim for the resulting username in the database. By default, this is the `email` claim. On my setup, emails and usernames are modifiable by users. That's why I use the `sub` claim as the username in other apps. For example, Grafana offers the `name_attribute_path` config value for their OAuth configuration. In alf.io I'd love a corresponding `openid.usernameClaim` config value.

**To Reproduce**
Steps to reproduce the behavior:
1. Follow the OpenID instructions using Keycloak: https://github.com/alfio-event/alf.io/blob/2.0-M4-2204/website/content/en/docs/Configuration/OpenID/_index.md
2. Enable the `openid` Spring profile
4. Run alf.io
6. Navigate to example.com/admin
7. 500 error without any logs (OpenID values are not properly set)
8. If above error is fixed, the OIDC provider (Keycloak) will most likely return an error since the requested scopes are actual claims that don't exist as scopes.

**Expected behavior**
Be redirected to Keycloak, being able to login, and be returned to the admin page.

**Desktop (please complete the following information):**
- OS: Ubuntu 20.04
- Browser: Brave
- Version: Keycloak v17.0.2, alf.io 2.0-M4-2204