feat: add sops

We've previously used SOPS for our secrets management and liked it. We
did, however, find the configuration was a bit annoying to do. In aid of
this, we've made a SOPS module that we find a little easier to make the
sort of configurations we want without creating so much mess.

We haven't set up scalpel/equivalent yet - we intend to avoid it if at
all possible. It isn't necessarily out-of-scope but it isn't included in
our current SOPS plans.

Change-Id: I35b9c7e94c12a4f1360833026efe06803d59626e
Reviewed-on: https://git.clicks.codes/c/Infra/NixFiles/+/725
Reviewed-by: Samuel Shuert <coded@clicks.codes>
Tested-by: Samuel Shuert <coded@clicks.codes>
diff --git a/.gitignore b/.gitignore
index e63526b..ee056bc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@
 
 result
 .direnv
+.sops.yaml