feat(base_layer): basic validations for proposals, proposal acceptances and amendments

[mrnaveira]

Description

• Added new base layer validations for:
  • Contract update proposals:
    • The corresponding constitution must be already published
    • No duplicated proposals (same contract_id and proposal_id) can be published
  • Contract update proposal acceptances:
    • The corresponding update proposal must be already published
    • No duplicated acceptances (same proposal and validator node) can be published
  • Contract amendments:
    • The corresponding update proposal must be already published
    • No duplicated amendments can be published for the same proposal
    • The updated_constitution ratified in the amendment must match exactly the one in the update proposal
• Added new unit tests for all the new validations
• Adapted an existing integration test for amendments due to the new validations
• Added ContractAmendment to the base layer database index (lmdb)
• Refactored some redundant helper code for validations

Motivation and Context

The base layer should perform basic validations (required prior data, no duplications and consistency) on update proposals, proposal acceptances and amendments

How Has This Been Tested?

• New unit test to check the new validation
• The existing unit and integration tests pass