Add Pluggable Transports and support for Partisan

[sahilpohare]

What this solves

Horde assumes Erlang distribution throughout — Node.list() for peer discovery, :erpc for remote
calls, and a single hardcoded Horde.NodeListener for membership. This makes Horde incompatible with
alternative transports like Partisan, which replaces Erlang distribution entirely with its own mesh
networking layer.

What it introduces

Horde.NodeListenerBehaviour

A use-based behaviour for building custom node listeners. The use macro injects a full GenServer
implementation with sensible defaults — Erlang distribution monitoring, handle_nodeup/handle_nodedown,
and set_members. The only required callback is make_members/1. All injected functions are
defoverridable.

Horde.Registry and Horde.DynamicSupervisor now accept {:auto, ListenerModule} as the :members
option to start a custom listener instead of the default Horde.NodeListener.

Horde.ClusterTransport behaviour

Abstracts the primitives Horde needs from the underlying cluster transport:

Callback	Purpose
members/0	Returns connected peer nodes
process_alive?/1	Checks whether a remote pid is alive
call/5	Invokes a function on a remote node

Two implementations ship out of the box:

Module	Backend
Horde.ClusterTransport.Erlang	Standard Erlang distribution (default)
Horde.ClusterTransport.Partisan	Partisan — :partisan_peer_service + :partisan_rpc

The :transport option on Horde.Registry selects the implementation. The chosen module is stored in
registry ETS metadata at startup and read on every lookup/2 — one local ETS read, no GenServer call on
the hot path.

Horde.NodeListener.Partisan

A ready-made Partisan node listener built on Horde.NodeListenerBehaviour. Uses
:partisan.monitor_nodes/1 for monitoring and :partisan_peer_service for membership.

{Horde.Registry,
  name: MyRegistry,
  keys: :unique,
  members: {:auto, Horde.NodeListener.Partisan},
  transport: Horde.ClusterTransport.Partisan}

Conditional compilation

Horde.ClusterTransport.Partisan and Horde.NodeListener.Partisan are wrapped in
Code.ensure_loaded?(:partisan_peer_service) guards — only compiled when Partisan is present. No optional
dep entry in mix.exs, no compile-time warnings in Partisan-free builds.

Methodology

• No breaking changes — all defaults preserve existing behaviour. :transport defaults to
  Horde.ClusterTransport.Erlang, {:auto, ListenerModule} is additive alongside the existing :auto.
• Hot path stays fast — transport is resolved once at init and stored in ETS metadata, never changes
  at runtime.
• Transport is stateless — just a module, no process or config storage beyond the single ETS write at
  startup.
• Existing tests pass — 104 tests pass. 3 pre-existing failures in
  ClusterTest/NetworkPartitionTest are unrelated flaky distributed tests.

[sleipnir]

Hi @sahilpohare, thanks for the PR, I'll review it soon.