Make overload in retry to take exception before pause provide

[ijsgaus]

WaitAndRetry(int, Func<int, TException, TimeSpan> timeout);

[reisenberger]

Broadly related to #177 .

At the moment, we are looking not to add further policy configuration overloads to Polly, due to the already too-numerous overloads. The forthcoming syntax changes proposed in #281 are intended to reduce the number of overloads by splitting onPolicyEvent-type delegates out of Policy configuration.

It could be good then to add this after the #281 syntax changes.

[reisenberger]

This feature is delivered in Polly v5.6.0 (will be released via nuget in next day or so).