Add OpenRPC example demonstrating x-* specification extensions

[BelfordZ]

Motivation

• Provide a concrete OpenRPC document that demonstrates usage of the bundled specification extensions so consumers can see how to apply x-transport, x-notification, and x-error-group in a real document.

Description

• Add a new example OpenRPC document at src/examples/example-openrpc-with-spec-extensions.json that uses x-transport on servers and x-notification / x-error-group on methods in a Wallet Service example.
• Export the new example through both ESM and CJS entrypoints by adding openrpcUsingExtensions to the examples object in src/index.mts and src/index.cts.
• Document the example location by adding an ### Example OpenRPC document reference to README.md that links to the new example file.

Testing

• Ran npm run build which completed successfully.

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Codex Task