pallet-multisig: Improve opaque call handling (#10060)

* pallet-multisig: Improve opaque call handling

Before the opaque call was just a type redefinition of `Vec<u8>`. With metadata v14 that was
breaking external tools, as they stopped looking at the type name. To improve the situation the
`WrapperKeepOpaque` type is introduced that communicates to the outside the correct type info.

* Cleanup

* Fix benchmarks

* FMT