TUN-8661: Refactor connection methods to support future different datagram muxing methods

The current supervisor serves the quic connection by performing all of the following in one method:
1. Dial QUIC edge connection
2. Initialize datagram muxer for UDP sessions and ICMP
3. Wrap all together in a single struct to serve the process loops

In an effort to better support modularity, each of these steps were broken out into their own separate methods that the supervisor will compose together to create the TunnelConnection and run its `Serve` method.

This also provides us with the capability to better interchange the functionality supported by the datagram session manager in the future with a new mechanism.

Closes TUN-8661

Author: DevinCarr

connection/connection.go

@@ -36,6 +36,13 @@ var (
 	flushableContentTypes = []string{sseContentType, grpcContentType}
 )
 
+// TunnelConnection represents the connection to the edge.
+// The Serve method is provided to allow clients to handle any errors from the connection encountered during
+// processing of the connection. Cancelling of the context provided to Serve will close the connection.
+type TunnelConnection interface {
+	Serve(ctx context.Context) error
+}
+
 type Orchestrator interface {
 	UpdateConfig(version int32, config []byte) *pogs.UpdateConfigurationResponse
 	GetConfigJSON() ([]byte, error)