IAM 776 Implement validation for `groups`, `schemas`, `identities` handlers

[BarcoMasile]

Description

This PR introduces the first implementation for the validation layer. Validation is delegated to each validatingFunction that handlers register through the RegisterValidation method. The process is run inside one middleware provided by the ValidationRegistry object.

Example:

  groupsAPI := groups.NewAPI(/*...*/)
  groupsAPI.RegisterValidation(vldtr)

Changes

• enahnced Validation errors with detailed errors for the UI to consume
• added tests
• api struct validators (3rd party) enhanced to let errors use the json field name to avoid discrepancy between fields used by UI and fields referenced by backend errors
• added validation constraint on URL Params for endpoints to forbid empty values
• added implementation of validation process for the following handlers
  • schemas
  • groups
  • identities

Missing

Tests for new validation functions for handlers.