Cleanup rolling stock supported signaling systems

[flomonster]

Description

The signaling system list should be considered as dynamic information loaded from "extension".

Editoast validates the supported_signaling_systems using an enum which is not valid.

What should be done?

1. Remove the enum: https://github.com/osrd-project/osrd/blob/5895b8726a4f1969fdcd922404ac7aa2e388dcb2/editoast/src/schema/rolling_stock/mod.rs#L268-L277
2. Replace where the enum was used by Vec<String>
3. Remove the feature unused field:

• Make a migration
• Adapt editoast
• Adapt core
• Adapt python osrd_schemas