Refactor DtlsTransport, remove internal ownership and raw pointers.

Refactors DtlsTransport to function as a stateless wrapper regarding
DtlsTransportInternal. It no longer owns or stores a raw pointer to the
internal transport. Instead, some of the formerly internal logic has
been moved to JsepTransport and test fixtures, where actual ownership
lies (as indicated by explicit calls to Clear()).

Key changes:
* Remove DtlsTransport's internal DtlsTransportInternal*
* Remove DtlsTransport::internal() accessor to avoid spreading use of
  this pointer.
* Update SctpTransport and PeerConnection to access the underlying
  transport via SctpTransportInternal or RtpTransport which is closer to
  where the actual ownership is.
* Refactored DtlsTransport to be passive regarding state updates;
  subscriptions are managed externally, together with ownership.
* Updated unit tests (DtlsTransportTest, SctpTransportTest,
  JsepTransportControllerTest) to align with the clarified ownership
  model, ensure that tests own the internal transport explicitly in the
  same way as production code does.
* Removed SctpTransportInternal::SetDtlsTransport as transport injection
  is now enforced via constructor.
* This enabled making the transport_ pointer const, more reliable
  and also unblock multi thread access, which will be needed down the
  line.

This simplifies DtlsTransport, removes quite a bit of raw pointer
wrangling (although some still remains), prevents potential
Use-After-Free issues by decoupling the wrapper's lifetime from the
internal transport.

Bug: None
Change-Id: I37f74d7945c6f8a69dc53d217ec1e6726a63d3e2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/429041
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46360}