chainfee: add a new interface method GetFeeMap

This commit adds a new method `GetFeeMap` to handle querying the web API
and parse the response into a map that's used by the fee estimator. In
doing so we can provide a deeper abstraction over the `WebAPIFeeSource`
interface.

lnwallet/chainfee/estimator_test.go

@@ -24,6 +24,10 @@ func (e mockSparseConfFeeSource) ParseResponse(r io.Reader) (map[uint32]uint32,
 	return e.fees, nil
 }
 
+func (e mockSparseConfFeeSource) GetFeeMap() (map[uint32]uint32, error) {
+	return e.fees, nil
+}
+
 // TestFeeRateTypes checks that converting fee rates between the
 // different types that represent fee rates and calculating fees
 // work as expected.