Add a module to validate a review

[kulpreet]

This module can be used by marketplaces to validate a discovered review. The validation is specified here - https://github.com/ChluNetwork/chlu-protocol/blob/master/chlu_protocol.md#validating-a-review

[fazo96]

List of validations that we need to perform (roughly in order):

Basic validation (no signing/encryption stuff, no blockchain stuff)

• [x] See PR #37

Blockchain validation

• [ ] verify presence on the blockchain if the review was not discovered from it and the library has access, this is assuming the review is discovered along with a reference to the transaction ID
• [ ] if we have information about the blockchain transaction, check that addresses and amounts are matching

Signing/encryption validation

• [x] if the RR is encrypted, decrypt review record using vendor public key
• [x] validate PoPR signature using pub key from vendor if the PoPR is signed

Also we should add an option to fail the validation if the signature or the encryption is missing

[kulpreet]

We need the PoPR to be signed. To achieve that we need to do the key setup part first. But for the first demo, lets put in a mock PoPR validation function that returns true.

[fazo96]

Updated comment with the validations that I split out from PR #37

[fazo96]

@kulpreet Closing this and moving the blockchain stuff to a new issue #85