fix: handle relative path to db

[dandanlen]

Pull Request

Replaces #4142

Ensures that the config root exists and that paths defined in the config file point to actual files where required (key files, mainly).

Tolerates both absolute and relative paths. Relative paths are resolved relative to the config root.

I had to shuffle some of the config test files around and add some dummy key files for the tests.

[codecov]

Codecov Report

Merging #4164 (bf86d6c) into main (6f5ec83) will increase coverage by 0%.
The diff coverage is 94%.

@@          Coverage Diff           @@
##            main   #4164    +/-   ##
======================================
  Coverage     72%     72%            
======================================
  Files        379     379            
  Lines      61670   61792   +122     
  Branches   61670   61792   +122     
======================================
+ Hits       44240   44348   +108     
- Misses     15138   15144     +6     
- Partials    2292    2300     +8     

Files	Coverage Δ
engine/src/settings.rs	87% <94%> (+1%)	⬆️

... and 5 files with indirect coverage changes

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