Use EventSource for SSE implementation

[mattt]

This PR updates HTTPClientTransport to use the EventSource package for processing server-sent events, replacing the ad hoc implementation with a more exhaustively tested dependency.

This PR also incorporates changes by @stallent in #91, specifically:

• Handling 405 response for streaming GET requests: If a server doesn't support streaming from the GET endpoint, it should return a 405. In response, the client cancels the streaming task.
• Add support for streams coming back from POST requests: This involves changing session.data(for: request) to session.bytes(for: request) in the send function and adding logic to process the SSE stream or JSON data from the response stream.