Update IWYU tool with a mapping file

Also apply IWYU to all .cc files in pc/, and correct BUILD file to match.
Note: Some files came out wrong when iwyu was applied. These are not included.

Bug: none
Change-Id: Ib5ea46b8fcc505414d0447cca7218ad3afc2e321
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/252280
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36064}

pc/dtls_transport.cc

diff --git a/pc/dtls_transport.cc b/pc/dtls_transport.cc
index e8d6ae9..c9f3279 100644
--- a/pc/dtls_transport.cc
+++ b/pc/dtls_transport.cc
@@ -19,7 +19,7 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/ref_counted_object.h"
-#include "rtc_base/ssl_certificate.h"
+#include "rtc_base/ssl_stream_adapter.h"
 
 namespace webrtc {