Add support for attribute constraints

[gaurav]

CAM-KP-API does not currently support any attribute constraints. It should be fairly straightforward to add a constraint system based around the five operators defined in TRAPI 1.3: '==', '>', '<', 'matches' and '==='.

https://github.com/NCATSTranslator/ReasonerAPI/blob/1e0795a1c4ff5bcac3ccd5f188fdc09ec6bd27c3/TranslatorReasonerAPI.yaml#L1093-L1123

However, since the ImplementationGuide for TRAPI 1.3 specifically allows us to report an error if any unrecognized constraint is received (https://github.com/NCATSTranslator/ReasonerAPI/blob/1e0795a1c4ff5bcac3ccd5f188fdc09ec6bd27c3/ImplementationRules.md#qnodeattribute_constraints). Therefore we won't be releasing attribute constraints with CAM-KP-API 0.1.3-trapi-1.3, but hope to implement that fairly soon.