Do we want to declare nested provider-specific properties in the config file?

In the section database-provider-specific properties it is described that databases are allowed to add provider specific fields to OPTIMADE dictionary fields, such as adding _exmpl_oxidation_state to the species dictionaries.
Do we want to support declaring such properties in the config file?
It may be useful, for example, for adding extra fields to the species dictionary, which may be needed for the implementing a OPTIMADE database for BIoexcel.

If so, what syntax should we use for the field name?
Do you think there will be a problem with a name like "species.oxidation_state" to declare that there is a provider specific property within species?

Start with the database-provider-specific properties section in optimade.rst and trace how the configuration file is documented or handled. Determine whether nested provider-specific properties should be declared and what field-name syntax would represent them; done requires a decided, documented approach.