Add client-side Streamable HTTP transport support

[halter73]

Adds client-side Streamable HTTP transport support. This is a follow up to #330. After this, I'll add a stateless mode for Streamable HTTP on the server.

Contributes to #157
Contributes to #158

[stephentoub]

Thanks.

[TomasHubelbauer]

Is it intentional that StreamableHttpClientSessionTransport is internal? I am trying to use it with a server that I made that uses the streamable HTTP transport mode with JSON mode not SSE: https://github.com/TomasHubelbauer/bun-mcp#http-streamable-transport

Edit: see #449