Add support for the GET /admin/realms/{realm}/users/count endpoint in the Admin API SDK

[kanpov]

Why is this needed: In order to determine, for example, the total amount of users in the system, which is crucial for pagination (whether there is a next page or a previous page, how many items in total etc.)

Referring to the Admin API documentation for version 25.0.0 of Keycloak, this is the endpoint:

[kanpov]

@NikiforovAll Made a proposal to add this endpoint in PR #120

[NikiforovAll]

Thank you for your contribution. Do you need this one, should I release the version/pre-release version?

[kanpov]

Thank you for your contribution. Do you need this one, should I release the version/pre-release version?

Yes, one of my projects currently needs user pagination which is why I contributed this. I'm planning on contributing more endpoints, but later