Add `path` field to StoreConfig which makes it possible to configure
where RocksDB is located rather than hard-coding the location to
`data` subdirectory.

This helps with setup where neard home directory and the storage live
on separate file systems.  Previously such configurations required
using of symbolic links or mounting storage inside neard home.  Both
of those solutions weren’t as clean as ability to point neard directly
at the right place.

This will become even more useful with planned cold storage which will
call for all archival nodes to use multiple disks for the data (SSD
for head data and HDD for archival data).

Fixes: near#6857