client implementation

[topherbullock]

Currently the gem only includes an MCP::Server implementation, but an end to end SDK implementation should also include a client.

Follow the same pattern for pluggable transports and implement Client and implement stdio and StreamableHTTP

[omenking]

Yep, why don't have a client implementation?

[atesgoral]

@omenking The PR linked above (#28) will do that.