Add Validate() methods for claim types

[mtmk]

Description

The Go implementation has comprehensive Validate(vr *ValidationResults) methods on every claim type and sub-component:

• OperatorClaims.Validate(), Operator.Validate()
• AccountClaims.Validate(), Account.Validate()
• UserClaims.Validate(), User.Validate()
• ActivationClaims.Validate(), Activation.Validate()
• AuthorizationRequestClaims.Validate(), AuthorizationResponseClaims.Validate()
• Export.Validate(), Exports.Validate(), Import.Validate(), Imports.Validate()
• OperatorLimits.Validate(), ExternalAuthorization.Validate()
• Plus subject validation, URL validation, signing key validation, etc.

The .NET library currently only has ExpectedPrefixes() and JWT header validation. A validation system would help users catch configuration errors before encoding.

Reference

Go implementation: validation logic spread across each claim type file and validation.go