Add implementedBy example

[BelfordZ]

Summary

Add a new OpenRPC specification extension: implementedBy.

This extension allows a Method Object to explicitly declare which logical participant(s) are responsible for handling a method. It enables clear modeling of bidirectional JSON-RPC APIs where both peers may receive requests or notifications.

The field is an array of role identifiers (e.g. ["server"], ["client"]).
If omitted, tooling SHOULD assume ["server"] for backward compatibility.

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Motivation

OpenRPC currently assumes a mostly server-handled API surface. In practice, many real-world JSON-RPC systems are bidirectional:

• servers call clients (callbacks)
• notifications flow both directions
• pubsub patterns are common

Without an explicit ownership signal, tooling must guess:

• which methods to route inbound
• which methods are callbacks
• which methods to mock or emit

This leads to ambiguity and inconsistent behavior across generators, mock servers, and documentation tools.

implementedBy makes method ownership explicit while remaining transport-agnostic and minimal.

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Design

• Added as a Method Object extension
• Type: string[] (free-form role labels)
• Common conventions: server, client
• No enums or hardcoded vocabulary
• No transport or lifecycle semantics introduced

Tooling can deterministically:

• accept inbound calls only for methods it implements
• expose outbound helpers for methods implemented by other participants
• group methods in documentation
• support bidirectional mocks

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Scope

This change strictly defines method ownership only.

It intentionally does not introduce:

• transport semantics
• routing rules
• pubsub lifecycle modeling
• new protocol behavior

Those concerns remain outside the scope of this extension.