keymgr: First steps towards path validation and permission checks

gabi-250requested to merge
gabi-250/arti:keymgr-mistrust into main

This adds a Mistrust param to the ArtiNativeKeyStore constructor and slightly changes its semantics (it now creates the keystore dir if it doesn't already exist, or, if it does, validates keystore_dir according to the specified Mistrust settings).

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