[fedcm] Refactoring: move delegation under its own directory

This CL moves the JWT signing and the SD-JWT parsing to its own
directory, as well as the network handling, so that it gets abstracted
away from most readers.

Change-Id: Ic3d16504fd19e10e32a08e9237e316d1eaaccfe4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6838828
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Sam Goto <goto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1500185}