Move candidates logic to SessionDescriptionInterface

...from JsepSessionDescription.

The `JsepSessionDescription` class was the sole implementation of the
`SessionDescriptionInterface` and still held the state and logic for
managing ICE candidates although all other state has already
been moved to `SessionDescriptionInterface`. This additional layer of
abstraction is now for the most part removed. `JsepSessionDescription`
still exists for compatibility reasons with downstream code.

`SessionDescriptionInterface` is no longer a pure abstract class and now
contains the default implementation for candidate management. A new
static `SessionDescriptionInterface::Create` factory method is
introduced to handle instantiation, and all existing factory functions
are updated to use it.

Bug: webrtc:442220720
Change-Id: I732938a958310c60a9a83592fb9b11e27778d77e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/409880
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45670}