Refine PolicyWrap configuration syntax to take parameters as interfaces

[reisenberger]

The current PolicyWrap configuration syntax in PolicyWrapSyntax.cs and PolicyWrapSyntaxAsync.cs pre-dates the introduction of interfaces: the parameters are the concrete classes Policy or Policy<TResult>.

Typing the parameters instead from the interfaces:

• ISyncPolicy
• ISyncPolicy<TResult>
• IAsyncPolicy
• IAsyncPolicy<TResult>

would:

• offer more precision (it wouldn't be possible to wrap a sync with an async policy, which would lead to a runtime error) (EDIT: tho the changes proposed in Cleanly formalise the separation of sync and async policies (was: option to unify them) #281 may make this superfluous)
• offer more flexibility - wraps could (in principle) use custom user ISyncPolicy implementations
• [EDITed to add] allow users to wrap policies where they had chosen to express the policies to be wrapped (when they defined them earlier), as interfaces. At the moment this is not possible, because the syntax only accepts the concrete classes.

[reisenberger]

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