feat: Add generic Protocol plugin and protocol interface

[jterapin]

Context

Adds the generic Protocol plugin, a Protocol interface base class, and a NoOpProtocol default: the mechanism for per-client protocol selection built on instance-level plugins (#347).

client = Service::Client.new(protocol: :rpc_v2_cbor)  # scoped to this client

The plugin resolves the :protocol option in before_initialize and installs build/parse handlers that delegate to the resolved protocol:

• Default (unset) — first registered protocol, or NoOpProtocol if none.
• Named (Symbol) — resolved from client_class.protocols.
• Custom object — used as-is (duck-typed; need not inherit Protocol).

Protocols are instances (not modules) so they can carry per-client state (e.g. query_compatible, codec instances).

Other Notes

• Dormant / define-only. Not wired into any weld, template, or generated client — the live rpcv2Cbor path is unchanged. Hooking the mechanism up and migrating rpcv2Cbor onto this interface follows in a later PR.
• Protocol is a base class our own protocols inherit and a documented interface custom objects merely satisfy — there is deliberately no is_a?(Protocol) check.
• Named protocols use Symbol to match the rest of the config surface.
• Stacked on feat: Restore instance-level plugins #347 — base that PR first; this PR's diff assumes it.
• The spec uses a test-only protocols registry (define_singleton_method) since the generated registry isn't restored until the mechanism is hooked up; a TODO marks its removal.