Refactor SignedExtension (#5540)

* Refactor SignedExtension

* Move DispatchInfo Associated type to Dispatchable
* Bound Call: Dispatchable
* Pass PostDispatchInfo to post_dispatch
* Pass DispatchInfo by reference to avoid clones

* Whitespace fix

Co-Authored-By: Tomasz Drwięga <[email protected]>

* Style changes from code review

Co-Authored-By: Kian Paimani <[email protected]>

* Only decalre in test mod to remove warning

* Deduplicate Call definition

* Bound frame_system::trait::Call by Dispatchable

* Introduce DispatchInfoOf type alias

* Whitespace fix from review

Co-Authored-By: Bastian Köcher <[email protected]>

Co-authored-by: Tomasz Drwięga <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>