[feature]: calculation of the success probability of a route outside without QueryRoutes

[feelancer21]

Is your feature request related to a problem? Please describe.
Today we had a discussion in a routing node group about whether it makes sense to have a cache for frequently used and successful paths outside of lnd so that we don't always have to using QueryRoutes and only update the route using BuildRoute. However, such a cache only makes sense if you can also update the success probability of the route outside of lnd.

Problem: BuildRoute does not currently return the probability of success. QueryProbability was marked as deprecated with 2d7fda2. And QueryMissionControl is quite cumbersome, as you would have to implement your own probability estimator outside of lnd.

Describe the solution you'd like
BuildRoute should return the success probability

Describe alternatives you've considered
QueryProbabilty should be revived. This would make it easier to calculate the probability for a route.