Add ChainTime associated type (#410)

* add HeaderTimestamp associated type

* use Header Timestamp

* rename HeaderTimestamp to ChainTime

* add unit test

* deal with clippy

* Apply suggestions from code review

Commit review suggestions

Co-authored-by: Tomasz Drwięga <[email protected]>

* code review

* cargo fmt

* get rid of additional test runtime

* unit test asserts against concrete import context

Co-authored-by: Tomasz Drwięga <[email protected]>