[Stacks 2.1] Support `coinbase-pay-to-alt-recipient` transactions

[zone117x]

See stacks-network/stacks-core#3164 which introduced the new tx type.

The encoding for the new format is fairly straightforward. Here's another PR https://github.com/hirosystems/stacks-encoding-native-js/pull/5/files that accomplishes the same change for decoding these txs. It's in rust, but the serialization format is more clear in this PR / codebase. It also includes a test vector w/ a serialized coinbase-pay-to-alt-recipient tx to test against.