staking: Flexible generation of reward curve and associated tweaks (#8327)

* Initial abstraction

* Alter rest of APIs

* Fixes

* Some extra getters in Gilt pallet.

* Refactor Gilt to avoid u128 conversions

* Simplify and improve pow in per_things

* Add scalar division to per_things

* Renaming from_fraction -> from_float, drop _approximation

* Fixes

* Fixes

* Fixes

* Fixes

* Make stuff build

* Fixes

* Fixes

* Fixes

* Fixes

* Update .gitignore

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/gilt/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update frame/gilt/src/mock.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Fixes

* Fixes

* Fixes

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>