jsonrpc: expose encoding and decoding functions (#114)

Expose `jsonrpc2.MakeID`, `jsonrpc2.EncodeMessage` and
`jsonrpc2.DecodeMessage` functions to allow implementing custom
`mcp.Transport`. 

Add an example that demonstrates a custom transport
implementation.

Fixes #110.

jsonrpc/jsonrpc.go

@@ -18,3 +18,22 @@ type (
 	// Response is a JSON-RPC response.
 	Response = jsonrpc2.Response
 )
+
+// MakeID coerces the given Go value to an ID. The value is assumed to be the
+// default JSON marshaling of a Request identifier -- nil, float64, or string.
+//
+// Returns an error if the value type was not a valid Request ID type.
+func MakeID(v any) (ID, error) {
+	return jsonrpc2.MakeID(v)
+}
+
+// EncodeMessage serializes a JSON-RPC message to its wire format.
+func EncodeMessage(msg Message) ([]byte, error) {
+	return jsonrpc2.EncodeMessage(msg)
+}
+
+// DecodeMessage deserializes JSON-RPC wire format data into a Message.
+// It returns either a Request or Response based on the message content.
+func DecodeMessage(data []byte) (Message, error) {
+	return jsonrpc2.DecodeMessage(data)
+}