add a type for arbitrary length Connection IDs, and parsing function

RFC 8999 allows Connection IDs of up to 255 bytes. Current QUIC versions
only use up to 20 bytes.

internal/protocol/connection_id_test.go

@@ -2,6 +2,7 @@ package protocol
 
 import (
 	"bytes"
+	"crypto/rand"
 	"io"
 
 	. "github.com/onsi/ginkgo"
@@ -105,4 +106,18 @@ var _ = Describe("Connection ID generation", func() {
 		var c ConnectionID
 		Expect(c.String()).To(Equal("(empty)"))
 	})
+
+	Context("arbitrary length connection IDs", func() {
+		It("returns the bytes", func() {
+			b := make([]byte, 30)
+			rand.Read(b)
+			c := ArbitraryLenConnectionID(b)
+			Expect(c.Bytes()).To(Equal(b))
+		})
+
+		It("returns the length", func() {
+			c := ArbitraryLenConnectionID(make([]byte, 156))
+			Expect(c.Len()).To(Equal(156))
+		})
+	})
 })