feat: add `/paras/head/included-candidates` and `/paras/head/backed-candidates` (#1166)

* feat: add /paras/head

* backed candidates endpoint

* finalize the backed-candidate logic

* fix grumble with return type

* fix types and complete /paras/head logic

* cleanup

* add test for parasHead, and add at field for responses

* add test for backed candidates

* add docs

* add inline docs

* rework the endpoints

* update docs

* remove mute mock data

* update endpoint descriptions

* leases to heads

* adjust inline comment

* add the endpoints to the latest e2e tests

* fix spacing