Refactor JsepTransportCollection to use std::vector for the transports

Changes the internal storage of transports in `JsepTransportCollection`
from `flat_map` to `std::vector`. Since the transport name is available
on the object itself, the map structure was redundant, and a vector is
generally more efficient for the typical number of transports.

Replaces the `Transports()` and `ActiveTransports()` accessors, which
constructed and returned `std::vector`s, with `ForEachTransport()` and
`ForEachActiveTransport()` methods that accept a callback. This avoids
unnecessary memory allocations during iteration and directly handing out
vectors of pointers to the transports.

Updates `JsepTransportController` to use the new iteration API and
adopts `absl::algorithm::container` helpers (`c_for_each`, `c_find_if`,
`c_any_of`) for cleaner and more compact implementation.

No-Iwyu: Will address iwyu issues in jsep_transport_collection.cc and jsep_transport_collection.h in a follow-up CL.
Bug: none
Change-Id: I8b59aea0d552c10e78d3109dd73859b4a4113fcc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427321
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46292}