Use absl::AnyInvocable instead of std::function in JsepTransportCollection and JsepTransport

This change updates `JsepTransportCollection` and `JsepTransport` to use `absl::AnyInvocable`
instead of `std::function` for callbacks.

This aligns with webrtc guidance:
https://webrtc.googlesource.com/src/+/refs/heads/main/g3doc/style-guide.md

This change was made by Gemini using the following prompt:
Use commit d471f7cf9a3e7900404ba5ef1 as a template for how to port each use of std::function in the `pc/jsep_transport_collection.h` and `pc/jsep_transport.h` files to absl::AnyInvocable.
Use absl::AnyInvocable<void() &&> for callbacks used once and absl::AnyInvocable<void()> otherwise.
Port usage one at a time and use `ninja -C out/Default` to see if you change compiles before moving to the next usage.
Use `tools_webrtc/gn_check_autofix.py -C out/Default` to update the BUILD.gn files.
Use `git cl format` to ensure formatting.
Use `git diff --name-only | xargs tools_webrtc/iwyu/apply-include-cleaner` to add include files needed.

When done, write a commit message in the same style as the commit above and acknowledge this was done by gemini and include this prompt. Finally, verify the commit message.

Bug: None
Change-Id: I69c1626da573b58df7422935b3d4616b33e3c352
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/425121
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#46193}