Add CachePath as a configuration option

[kylecarbs]

This allows a configurable cache path to store postgres archives. Callers can centralize the location of all data stored by this library with this change.

[kylecarbs]

@coder we plan on using this to allow for a simpler setup of coder/coder. We want to reduce the files that we touch to the users' single CLI config directory.

[fergusstrange]

Hey @kylecarbs apologies for the slowness. It looks like you've significantly dropped coverage with this change. We'd need to at least maintain the current level in order to progress.