Add Domain (tenant) CRUD to Client SDK v2

Add v2 client and DTOs for Domain (tenant) management.

### DTO (common/dto/manager/domain/)
- request.py: CreateDomainRequest, GetDomainRequest, SearchDomainsRequest (filter/order/pagination), UpdateDomainRequest, DeleteDomainRequest, PurgeDomainRequest
- response.py: DomainDTO, CreateDomainResponse, GetDomainResponse, SearchDomainsResponse, UpdateDomainResponse
- types.py: DomainFilter, DomainOrderField, DomainOrder

### V2 Client (client/v2/domains/domain.py)
- DomainClient(BaseDomainClient) — create, get, search, update, delete, purge methods
- Register domain property in BackendAIClientRegistry

### Reference
Refer to v1 func/domain.py GQL-based list/detail/create/update/delete/purge. v2 uses REST-based typed_request pattern.

JIRA Issue: BA-4532