Schema Creation should Reject `SignatureRequired` for OnChain and IPFS Payload Locations

[wilwade]

Currently it is possible to create a schema with the setting SignatureRequired when also using the PayloadLocation of OnChain or IPFS.

Expected: Schema creation and proposal for creation should reject a schema setting that does not apply to the selected PayloadLocation

As a developer creating a schema (or someone reading a schema), I want to make sure that I do not incorrectly think that the SignatureRequired will do something for Message Storage based schemas, when it does not.