Allow to set max_total_routing_fee_msat

[tnull]

LDK 0.0.117 introduces a new parameter for limiting the maximum total routing fee for any given payment.

While LDK sets a default value of 1% of the payment value plus 50_000 msat, we should allow the user to override this field.

However, it's not immediately clear how to do it best: adding a general default setting to Config seems a bit rigid, however, we'll probably don't want to clutter the interface by adding it as an optional parameter to all send_ calls.

However, we could consider introducing a PaymentParameters struct that would allow users to override multiple values. If we go this route, we should consider how much of upstream's RouteParameters we want to replicate here.

[tnull]

Done in #336.