sync katzenpost v0.0.48

[xendarboh]

katzenpost/katzenpost@v0.0.43...v0.0.48

Update to latest katzenpost release.

Additionally, refactor pki/ customization to support easier integration of future upstream changes.

Replaces #93

[xendarboh]

The PR for this issue (#105) and this diff in particular may be useful for future updates.