Consolidate Supervisor filesystem pathing code

[christophermaier]

We have code for determining the appropriate filesystem paths for several parts of the Supervisor, split across several locations. For consistency, we should consolidate all these.

For example:

• FsCfg (if we consolidate to pure functions, it's not clear that we even need FsCfg).
• sup_protocol::sup_root (Not sure why this is in sup_protocol; also, the custom_state_path argument of this function is only used in tests, which ends up needlessly cluttering our real code)
• hab_sup::fs

There may be additional places where things could be consolidated; this is just from a cursory survey.

Note that there is also a core::fs module that has some similar functions; these should deal with non-Supervisor filesystem paths, though. We should base this refactoring on that module where appropriate, but endeavor to keep it restricted to purely Supervisor concerns.

[stale]

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. We value your input and contribution. Please leave a comment if this issue still affects you.