Add APIs to classify channel-related transaction types

[tnull]

It would be great if we could add APIs that would allow the user to determine which 'type' of transaction any given transaction would be related to channel operation.

Especially when it comes to splicing it would be helpful if a user could check whether a particular Txid is 'known' to LDK and retrieve whether its a channel funding/channel close/splice in/splice out transaction.