Reland "Allow setting a DtlsTransportFactory in PeerConnectionDependencies"

This is a reland of commit d81865fb38bbdf7ba19f713338cbd8fdf25d513b

Original change's description:
> Allow setting a DtlsTransportFactory in PeerConnectionDependencies
>
> The JsepTransportController::Config already supports it, so this is
> just exposing up to PC clients.
>
> Note this required some work to avoid a circular dependency - changing
> to use forward declarations in dtls_transport_factory.h
>
> Bug: chromium:443019066
> Change-Id: I3b65fe36d88e25e40f54bafe858851aff723b16b
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/428281
> Commit-Queue: Tony Herre <herre@google.com>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#46317}

Bug: chromium:443019066
Change-Id: Ib446cef413c89dd83b7b5b1652b7f525f2bc7f0d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/428900
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46324}