Add functions for multi-miner contract to call register asset functions

[kantai]

register-new-nft-contract and register-new-ft-contract need to have multi-miner methods for invocation by federated subnet miners.

I think these should do the same threshold checks that the block-commit method does.

[a3slade]

Needs to also update these:

• Proposals: https://github.com/hirosystems/stacks-subnets/blob/master/src/chainstate/stacks/miner.rs#L142
• Sign and verify functions: https://github.com/hirosystems/stacks-subnets/blob/master/src/chainstate/stacks/miner.rs#L2294
• rpc to collect signatures: https://github.com/hirosystems/stacks-subnets/blob/master/src/net/rpc.rs#L931