feat: add Azure Key Vault parameter provider

[fedemaleh]

Summary

Adds the Azure Key Vault parameter provider (store, retrieve, delete).

Warning

Draft — not ready to merge. This provider has not been fully tested yet. Opened as a draft to track review alongside the HashiCorp Vault provider, which ships in a separate PR.

Split out from feature/azure-keyvault-hashicorp-vault-providers.

What's included

• Provider scripts: parameters/providers/azure-key-vault/{setup,store,retrieve,delete}
• Install specs (specs/): provider specification, per-instance configuration, API key, agent notification channel
• Docs: docs/architecture.md
• BATS tests under parameters/tests/providers/azure-key-vault/
• CHANGELOG entry

Test plan

• BATS unit tests
• Manual validation of store/retrieve/delete against an Azure Key Vault instance
• End-to-end install via specs

Note: this branch is based on main. Once the HashiCorp Vault PR merges, expect a trivial CHANGELOG conflict to resolve here.