Add MACsec policy, MKA policy and keychain resource modules

[ed6869]

Summary

pyaoscx currently has no SDK classes for the AOS-CX security configuration exposed under system/macsec_policies, system/mka_policies and system/keychains (including the nested per-keychain key collection). These resources are required to build Ansible modules for MACsec / MKA / keychain management in the arubanetworks.aoscx collection, which follows the SDK-backed module pattern (from pyaoscx.X import Y).

Requested classes

• MacsecPolicy — system/macsec_policies, indexed by name (scalar + nested bypass / cipher_suites dicts).
• MkaPolicy — system/mka_policies, indexed by name (scalars + keychain URI reference + write-only cak/ckn).
• Keychain — system/keychains, indexed by name.
• KeychainKey — nested system/keychains/{name}/keys, indexed by key_id (child object of Keychain).

REST API version

These resources require REST API v10.16 (they return HTTP 400 below v10.10). pyaoscx ships up to v10.09 only, so v10.16 API support needs to be added as well. Note this overlaps with the v10.16 support proposed in #72 / #73 (Port Access family) — whichever merges first, the other will need a trivial rebase.

Environment

• AOS-CX 6300, firmware FL.10.17.1001, REST API v10.16.