WIP: New way to schedule custom ComboHypos [ATR-23484]

For some purposes, e.g. tag-and-probe chains, we generally need more flexibility to place ComboHypo instances such as those implementing topo selections with knowledge of the full chain, rather than in fixed steps. This is probably addressed well by generalising the usage of the scheduling code used for cross-signature ComboHypos in combined chains.

To do:

• Switch over signature-specific topo configuration code to use the general mechanism
• Allow the ComboHypo code to attach topos to steps other than the last