Add KeycloakRealmResource and AddRealm method

[paulomorgado]

Description

This PR introduces a new AddRealm method in the KeycloakResourceBuilderExtensions class, enabling the addition of Keycloak Realm resources to the application model. The method constructs a KeycloakRealmResource using an IResourceBuilder<KeycloakResource>, a realm name, and an optional realm name parameter.

Additionally, a new KeycloakRealmResource class is defined, which includes properties for various Keycloak endpoints and their expressions, along with a constructor and detailed XML documentation.

Changes to PublicAPI.Unshipped.txt reflect the inclusion of the new method and class in the public API.

Fixes #5092

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Is this introducing a breaking change?
      • Yes
        • Link to aspire-docs issue (please use this breaking-change template):
      • No
        • Link to aspire-docs issue (please use this doc-idea template):
  • No

[davidfowl]

No sample or tests?