Extra requirements for claimants (#1091)

* Introduce mandatory statement signing into claims

* Introduce SignedExtension

* Tests passing

* Bump runtime version

* Bump version, fix test

* Test for validate

* Another couple of tests

* Enable PrevalidateAttests on Polkadot

* Enable PrevalidateAttests on Polkadot

* Fix build

* Fixes

* More fixes

* Fix bench tests

* Fix & test Preclaim clobbering.

* Fix for errant claim logic

* Add test

* Update tests, always use Vec as input

* mint_claim can add signature, some_benchmarks

* Add claim with statement test

* finish benchmarks

* put the correct number of claims with benchmarks

* fix compiler warning

* Update weights

* Weight comments for validation

Co-authored-by: Shawn Tabrizi <[email protected]>