Add support for Streamable HTTP transport

[haroldadmin]

This PR adds support for the new Streamable HTTP transport to this Fastify plugin.

• Support for both stateless and stateful sessions in a single plugin
• Tests for both implementations
• New sample server demonstrating how to use the streamable HTTP transport
• Breaking: the Sessions class is now generic on the Transport type exported from the SDK.

TODO:

• I need to do some more work to ensure sessions are cleaned up properly when using this transport, but things seem to work well for now.
• The plugin does not support event stores yet, expect that to be added in a future release.