Apply TURN server checks to UDP redirects

A 300 (Try Alternate) response to a TURN allocation over UDP makes the
port reuse its socket and send a new Allocate request to the alternate
server directly, without going through PrepareAddress(). That skipped
two checks that apply to every other way of connecting to a TURN server:
the check for disallowed ports and the Local Network Access permission
request.

- Reject redirects to disallowed ports in SetAlternateServer(), next to
  the loopback and unspecified address checks, so that all protocols are
  covered. Redirects over TCP and TLS to such ports were already blocked
  by PrepareAddress(), but are now rejected before reconnecting.
- Request Local Network Access permission for the alternate server
  before sending the Allocate request over UDP, using a helper shared
  with PrepareAddress().

Bug: chromium:502821327
Change-Id: Ib2f06c9bfc51fbf6dadddbd701774e33d4c77ce8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507220
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48807}