[Feat]: Samples for implementing RPC extension

### Is your feature request related to a problem? Please describe.

Unless I'm missing something, I've been unable to find a complete extension example that implements a RPC method extension.

### Describe the solution you'd like

An example that follows best practices for implementing a custom RPC method extension and what is needed from a client and server perspective.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look for existing RPC method examples in the repository, likely in directories like 'examples' or 'extensions'. Review the A2A protocol documentation to understand RPC extension structure. Create a sample that includes both client-side invocation and server-side implementation, ensuring it follows the project's best practices for extensions.

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