waddrmgr: add TODO to determine proper address type of imported key
waddrmgr: add TODO to determine proper address type of imported key

For key scopes which have an address schema where the external and
internal branches differ, we always assume that imported keys use the
external address type defined in the scope's address schema. This may
not always be the case however, and should be handled correctly.
Ideally, we generate two addresses per imported key (only if the
external and internal address types differ) and scan for both in the
chain.

btcsuite/btcwallet,btcsuite/btcwallet