Introduce storage types, to builds abstraction on storage (to be used in...

Introduce storage types, to builds abstraction on storage (to be used in pallet attribute macro) (#7278)

* introduce storage types

* fix line width

* improve doc

* typo

* Apply suggestions from code review

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

* remove redundant function

* some more doc improvment

* disallow: some value on empty with option query

tests are update to still test default value is used when it is supposed
to be used.

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