Consolidate media content setting in BaseChannel

Move the implementations of SetLocalContent_w and SetRemoteContent_w
from the VoiceChannel and VideoChannel subclasses into the BaseChannel
class. This consolidation reduces code duplication and simplifies the
maintenance of SDP push-down logic across different media types.

To facilitate this change, the following modifications were made:
- Introduced std::variant to store media-specific sender and receiver
  parameters within BaseChannel.
- Updated the BaseChannel constructor to require a MediaType, allowing
  the unified methods to branch logic for audio or video as needed.
- Enhanced RTCError::InvalidParameter to support optional descriptive
  messages, providing clearer feedback when parameter settings fail.
- Cleaned up the ChannelInterface by removing AsVideoChannel and
  AsVoiceChannel helper methods, favoring the use of specialized media
  channel accessors.

Bug: webrtc:42224170
Change-Id: Ida66f61c12e992fed872a744bd412a3dc0c24f45
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/466020
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47539}