Migrate config.json from config-dir to backend (#6195)

This PR is the first set of changes to move the config
to the backend, the changes use the existing `config.json`
allows it to be migrated such that we can save it in on
backend disks.

In future releases, we will slowly migrate out of the
current architecture.

Fixes #6182

.gitignore

@@ -22,3 +22,4 @@ parts/
 prime/
 stage/
 .sia_temp/
+config.json