Add functionality to `Selector` to get the latest selector of a given type

[nategraf]

There are currently various places where we reference the latest selector of a given type, e.g. Selector::Groth16V2_0 or Selector::SetVerifierV0_6. We should provide a Selector::groth16_latest() function and a Selector::inclusion_latest() and do a find and replace for these references, as they will become out of date.

[linear]

WEB3-453 Add functionality to `Selector` to get the latest selector of a given type