Move JsepTransportController callbacks to Config

This refactors JsepTransportController to accept event callbacks via its
Config struct at construction time, replacing the previous pattern of
exposing Subscribe* methods and using rtc::CallbackList.

The internal signals are replaced with absl::AnyInvocable fields in the
configuration. This simplifies the initialization flow and removes the
dependency on CallbackList.

Additionally, this updates callback signatures to use absl::string_view
instead of std::string for transport names where applicable.

No-Iwyu: One header include will be removed in an upcoming cl.
Bug: webrtc:360058654
Change-Id: I7e1b882b5acd1a4324664c022e16f0482ec424a5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427160
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46257}