Move TCP candidate encoding constants to candidate.h

The constants for RFC 6544 TCP candidate encoding rules (`DISCARD_PORT`,
`TCPTYPE_ACTIVE_STR`, etc.) were defined in `p2p/base/port.cc`.

These constants are used by other targets, such as `webrtc_sdp`, and
defining them in port.cc contributed to a cyclical dependency with the
`Candidate` class.

Since these constants define properties of a `Candidate`, this commit
moves them to `api/candidate.h` to resolve the dependency and improve
co-location.

Bug: webrtc:12330
Change-Id: I205b46e99bc94997dc5845ed1c18f219970eb867
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/411181
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45788}