feat(base_layer): basic checkpoint validation

[mrnaveira]

Description

• Removed unnecessary validation about required contract definition in checkpoints
• Added checkpoint validation of existing prior contract constitution
• Added checkpoint validation of checkpoint signer public keys being included in the constitution committee
• For convenience, implemented IntoIterator trait for references of CommitteeSignatures

Motivation and Context

The base layer must perform some basic validations on checkpoints:

• The constitution must have been published
• The checkpoint signers public keys must be included in the constitution committee

How Has This Been Tested?

The new unit tests for the new validations pass