feat(base_layer): validation of committee membership in contract acceptances

[mrnaveira]

Description

• Created a new TxDanLayerValidator struct to encapsulate all DAN layer consensus validations
  • Created a submodule specific for contract acceptances, more submodules can be added for other validations (contract definition, contract constitution, etc.)
  • Created a helper submodule for functionality that is expected to be reused for future validations
• Modified the mempool to include the new TxDanLayerValidator
• New variant of validation error (DanLayerError) to represent DAN layer consensus validation errors
• Adapted the existing integration test for contract acceptances, by specifying the validator node identity file (fixtures/validator_node_id.json) so it's always present in the contract constitution

Motivation and Context

• The base layer should only allow contract acceptance transactions of validator nodes that are defined as committee members in the contract constitution
• This PR is the first base layer validation of the DAN layer, so it defines the structure for all future DAN validations

How Has This Been Tested?

• New unit test to check membership of a validation node in a contract constitution committee
• The existing unit and integration tests pass

[sdbondi]

utAck - thanks for writing tests. Looks like a cucumber test related to this is broken, otherwise will approve.