Add latest organization API endpoints

[societ-campbellwray]

Summary

• add the missing latest Keycloak Organizations endpoints, including organization groups, invitations, identity-provider groups, member groups, and organization count
• extend organization-related models to match the current OpenAPI response shapes
• keep the changes additive and preserve existing method signatures

Closes #76

Validation

• OpenAPI Organizations route comparison: 36/36 operations covered, no missing or extra verb/path matches
• Organization-related model property comparison against the latest OpenAPI schema
• dotnet build Keycloak.Net.Core.sln