Implementer's Guide: Incorporate HRMP to TransientValidationData (#1588)

* Add a note about time of collection of TransientValidationData

* Add HRMP digest and dmq length to TransientValidationData

* Add a note that the vector in hrmp digest is never empty

* Add hrmp watermark to TransientValidationData

* Add HRMP egress limits

* Incorporate the latest dispatchable upward messages changes.

* Update candidate.md

* Update candidate.md docs

* Fix wording

* Delete assignmets.md

* Update roadmap/implementers-guide/src/types/candidate.md

Co-authored-by: Bastian Köcher <[email protected]>

* Extract HrmpTransientValidationData and add additional data.

* Some clarifications.

* Introduce HrmpAbridgedOpenChannelRequest

* Update roadmap/implementers-guide/src/types/candidate.md

Co-authored-by: Bernhard Schuster <[email protected]>

* Fix typo: egress->ingress

* A note about sorting

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Bernhard Schuster <[email protected]>