Add via parameter on join room requests as per MSC4156 (#3441)

This implements
[MSC4156](matrix-org/matrix-spec-proposals#4156)
which will ship with Matrix 1.12 and is the continuation of
matrix-org/dendrite#3438.

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [ ] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>
Co-authored-by: Till <2353100+S7evinK@users.noreply.github.com>

clientapi/routing/joinroom.go

@@ -36,9 +36,16 @@ func JoinRoomByIDOrAlias(
 		Content:       map[string]interface{}{},
 	}
 
-	// Check to see if any ?server_name= query parameters were
-	// given in the request.
-	if serverNames, ok := req.URL.Query()["server_name"]; ok {
+	// Check to see if any ?via= or ?server_name= query parameters
+	// were given in the request.
+	if serverNames, ok := req.URL.Query()["via"]; ok {
+		for _, serverName := range serverNames {
+			joinReq.ServerNames = append(
+				joinReq.ServerNames,
+				spec.ServerName(serverName),
+			)
+		}
+	} else if serverNames, ok := req.URL.Query()["server_name"]; ok {
 		for _, serverName := range serverNames {
 			joinReq.ServerNames = append(
 				joinReq.ServerNames,