Load credentials from files, rather than hard-coding into config

[jml]

Motivation is to allow easy use of compare-revisions with Kubernetes Secrets.

This groups together several changes which are hard to separate without introducing broken builds:

• We now support using SSH keys as credentials for syncing Git repositories

• There is a new secrets clause in the config file, which allows users to define secrets by name and provide files which have the credentials

• ImageConfig is no longer parametrized, but replaced with two concrete types: ImageConfig' for user-provided config, and ImageConfig for the fully realised & validated version

• We use Git.URLWithCredentials everywhere we were previously using Git.URL to identify a repository.

[jml]

Merging without review, although would really appreciate a post hoc one.

I made this branch mostly while really really tired over the course of weeks. I don't have the capacity to do a proper review myself.