Add ARO HCP proposal for CAPZ support

Adds comprehensive proposal document for implementing Azure Red Hat
OpenShift Hosted Control Plane (ARO HCP) cluster provisioning using
Cluster API Provider Azure (CAPZ).

The proposal covers:
- Architecture and design for ARO HCP integration with CAPZ
- ASO resource definitions and CAPI contract implementation
- KeyVault and encryption key management
- Security considerations and alternatives

This document serves as the technical foundation for ARO HCP CAPZ
development and provides guidance for customers deploying
ARO HCP clusters via CAPZ.

Author: marek-veber