chore(nodejs): extract transport and buffer from sender, add back support for core http

[glasstiger]

Extracting the transport layer from Sender, and introducing the SenderTransport interface with different implementations:

• TCP transport: uses a socket with ILP over TCP
• HTTP transport: uses node's core http module with ILP over HTTP
• HTTP Undici transport: uses the undici module with ILP over HTTP

Also extracting the buffer from Sender into a class SenderBuffer.
Now the Sender is mostly just wiring between a transport and a buffer, and responsible only for auto flushing.
In fact, you could create your own transport and buffer, and use them without a Sender if you are calling flush() manually.
You could also create your own pools of buffers and transports, the transports could be even different types, and just flush your buffers when needed using one of the transports from the pool.

Also refactored and improved tests.
Introduced more types, such as Logger, ExtraOptions, TimestampUnit and MockConfig.
Fixed any and unknown type warnings.