Apply include-cleaner to api direct files (2/2).

This is a follow up for https://webrtc-review.googlesource.com/c/src/+/360680.

* Adding some missing <optional> include.
* Adding a IWYU pragma to force keeping an include.

Note that I've added the CQ bot 'iwyu_verifier' to ensure the repo stays clean. It is still work in progress and it currently needs to be triggered manually.
FYI I used these command line to run iwyu:
> for i in api/*.cc; do ./tools_webrtc/iwyu/apply-include-cleaner $i; done
> for i in api/*.h; do ./tools_webrtc/iwyu/apply-include-cleaner $i; done

Change-Id: Ie7036d08edbb6884f2b35eb9d69646757d662390
Bug: webrtc:42226242
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/362440
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#43054}

api/audio_options.cc

@@ -10,6 +10,7 @@
 
 #include "api/audio_options.h"
 
+#include <optional>
 #include <string>
 
 #include "api/array_view.h"

api/data_channel_interface.cc

@@ -11,6 +11,7 @@
 #include "api/data_channel_interface.h"
 
 #include <cstdint>
+#include <optional>
 #include <string>
 
 #include "absl/functional/any_invocable.h"

api/jsep.cc

@@ -11,6 +11,7 @@
 #include "api/jsep.h"
 
 #include <cstddef>
+#include <optional>
 #include <string>
 #include <vector>
 

api/rtp_transceiver_interface.cc

@@ -10,6 +10,8 @@
 
 #include "api/rtp_transceiver_interface.h"
 
+#include <optional>
+
 #include "api/rtc_error.h"
 #include "api/rtp_transceiver_direction.h"
 #include "rtc_base/checks.h"

api/sctp_transport_interface.cc

@@ -10,6 +10,7 @@
 
 #include "api/sctp_transport_interface.h"
 
+#include <optional>
 #include <utility>
 
 #include "api/dtls_transport_interface.h"

api/webrtc_key_value_config.h

@@ -12,6 +12,6 @@
 
 // TODO(bugs.webrtc.org/10335): Remove once all migrated to
 // api/field_trials_view.h
-#include "api/field_trials_view.h"
+#include "api/field_trials_view.h"  // IWYU pragma: keep
 
 #endif  // API_WEBRTC_KEY_VALUE_CONFIG_H_