Add Http2Origin option to force HTTP/2 origin connections

If `http2Origin` is set, it will set `ForceAttemptHTTP2` in the transport config of the `OriginService`.

config/configuration_test.go

@@ -139,7 +139,8 @@ var rawConfig = []byte(`
 			"ports": [443, 4443],
 			"allow": true
 		}
-	]
+	],
+	"http2Origin": true
 }
 `)
 
@@ -188,6 +189,7 @@ func assertConfig(
 	assert.Equal(t, true, *config.NoTLSVerify)
 	assert.Equal(t, uint(9000), *config.ProxyPort)
 	assert.Equal(t, "socks", *config.ProxyType)
+	assert.Equal(t, true, *config.Http2Origin)
 
 	privateV4 := "10.0.0.0/8"
 	privateV6 := "fc00::/7"