Use a keyed hash for ICE candidate foundations

The foundation of a candidate was the CRC32 of the candidate type, the
base address, the protocols and the ICE tie-breaker. The tie-breaker is
sent to the remote peer in connectivity checks, so the remote peer could
recover the base address of a candidate from its foundation by brute
force, also when the address is hidden behind an mDNS name. Adding a
secret to the CRC32 input would not be enough, since CRC32 is linear and
foundations with known inputs, such as those of relay candidates, could
be used to cancel it out.

Compute the foundation as a truncated HMAC-SHA256 instead, keyed with a
random key that is generated once per process and never sent anywhere.
Equal inputs still produce equal foundations within a process and the
format (a decimal 32-bit number) is unchanged, but foundations are no
longer comparable across processes.

Bug: chromium:504578795
Change-Id: If07aee5410d5d29d3a74412b44f9704725e73788
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507200
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48803}