Implementer's Guide: Demote HRMP channel management to regular dispatchables

[pepyakin]

Demote the HRMP channel management upward message kinds to regular entrypoints.

First reaction is that these entrypoints should go into the Router module.

I think it might also imply changing ValidationData structs. The reason is the following. Currently if the collator provides invalid data to the PVF and then PVF does something nonsensical then the candidate is rejected and collator misses out. With dispatchables the candidate becomes valid and we need to pass the required data through the ground truth (if any) to inform the PVF. On the other hand, we may want to remove some stuff from the TransientValidationData.

Related #1702