Consider adding a json schema to represent a policy

[ahouseholder]

• offshoot of The JSON for SSVC "options" splits out keys into individual records #576

Describe the solution you'd like

• Do we need a separate schema for policies?

• Should policies be versioned?

• Decision Point Group schema exists

• Outcome schema does not, but would be prerequisite for this - see Need json schema for outcome groups #589

• policy schema would describe the mapping of a DPG -> O

• similar to Provisioned schema?-

Describe alternatives you've considered

Possibly adapt the Provisioned schema for this

[ahouseholder]

Version might be an optional field in the schema, since we don't intend to version policies, but we want to encourage SSVC users to consider versioning their own policies