keymgr: Validate ArtiPaths, replace HsClientSpecifier with generic ArtiPathComponent

Note: this builds on top of !1256 (merged). This should be reviewed starting from keymgr: Define ArtiPathComponent.

This replaces HsClientSpecifier with ArtiPathComponent (they both serve the same purpose except ArtiPathComponent is more generic and can be reused for other the other specifiers we'll need to implement).

This also addresses the TODOs regarding ArtiPath validation.