Use custom snapshots

[neongreen]

This uses Stack's custom snapshot feature (https://docs.haskellstack.org/en/stable/custom_snapshot/). A custom snapshot extends the set of packages in the resolver and can be used from any other project (including other Wire repos). No more copying parts of stack.yaml from one project to another.

Custom snapshots are stored in ~/.stack. This has several benefits:

• They survive stack clean. (Well, Git deps have to be cloned anyway because of a bug in Stack, but they won't be recompiled, only recloned, and for the biggest deps I've switched to using tarballs so that should be faster.)

• They survive branch switches (i.e. if different package versions are used in different branches, no recompiling is necessary when switching between the two).

• We no longer need to cache both .stack and .stack-work on CI.

A thing to keep in mind is that snapshots are immutable. A snapshot can be based on the previous version of the snapshot, but Stack takes a bit of time to symlink everything from the older snapshot to the newer one, so having a new snapshot every day would be inadvisable. For that reason packages that change frequently (like hscim) should remain in stack.yaml.

[neongreen]

Should be updated after the SCIM PR is merged