Clarify whether the constraints property should be mandatory

[decentralgabe]

The constraints property in input descriptors is mandatory; however, all properties in the property are optional. This seems to introduce a case which necessitates an empty constraints property exists.

Should the language be updated to....

1. Make constraints optional (what's the use case - just checking for format?)
2. Keep constraints required and specify that at least one sub-property must be present

[kimdhamilton]

Making this MAY -> does input_descriptors need to be MAY?

[decentralgabe]

@kimdhamilton no - since there is value on selecting a credential on format along (from @Sakurann)

[dif-admin]

We need to add format to input evaluation

[dif-admin]

Decision from 3/21: Keep it MUST, clarify in input evaluation