IF: set_finalizers updates

[heifner]

• Add finalizer_set to block_header extension
• Add last_proposed_finalizer_set_generation to block_header_state
• Add set_finalzers host function to eos-vm-oc/intrinsic_mapping
• Add to/from variant tests for bls_public_key
• Add activate protocol feature instant finality host function set_finalizers unittest.

Note: snapshot tests are failing because of block_header_state addition of last_proposed_finalizer_set_generation. I think the snapshot test should be fixed as part of #1558. I've commented it out and added a warning for now.

[ericpassmore]

Note:start
group: IF
category: CONSENSUS
summary: Updated block header state adding last_proposed_finalizer_set_generation and adding finalizer_set to block header extension. Created host function and protocol feature set_finalizers.
Note:end