feat: Initial Client

[cbrit]

Would like to get feedback on the direction I've gone so far, namely

1. The Client calls the Transport, as opposed to the the pattern used on the server side.
2. Having a client implementation per transport type (StdioClient, SseClient) as opposed to one type that owns a T such that T: Transport, where Transport is some trait that doesn't exist right now. This draft only has the initial work of a StdioClient, nothing for SSE yet.
3. Is switching to using generated types in offeryn-types out of scope for this work?