Decide on the approach for the non-standard InChI/InChIKey support

[vaitkus]

The InChI/InChIKey defines several chemical structure description layers that are not used in the Standard InChI (e.g. such as the Fixed-H or Reconnect layers). As a result, the Standard InChI do not discern between different tautomers or different metal coordination spheres with same components but different connectivity to the metal. Since for some databases this distinction might be quite important, it makes sense to also support the non-standard InChI generated using a different set of InChI generation options. Two approaches towards this support were discussed during the OPTIMADE 2025 workshop:

1. Explicitly define new properties that have a fixed non-standard properties. For example, the _cheminfo_fixed_h_inchi could contain InChI strings which are generated with the Fixed-H option enabled in addition to the standard ones thus permitting to discern between different tautomers.
2. Introduce a more generalised _cheminfo_inchi field that also somehow specifies the InChI options used to generate this InChI (e.g. as metadata).

The are currently no agreement on what the best approach would be.

[merkys]

An interesting blocker for command line option enumeration is that Linux and Windows use different conventions for CLI options, -- vs. \ accordingly. Thus a solution to make these OS-agnostic has to be found.