added support for intercepting methods that return IAsyncEnumerable

[ArnaudDebaene]

Hello JSkimming,

Here is a 1st implementation of adding support to IAsyncInterceptor in order to support mehods that return IAsyncEnumerable<>

Note that:

• I have NOT implemented support in AsyncInterceptorBase, because I am not certain about what should be done in that case... (simply call InterceptAsync<T> ? Add a new abstract method in AsyncInterceptorBase?)

• I have implemented only a few unit tests, nothing concerning exception handling.

For now, it is more a "proof of concept" that anything else.

PS : I had to remove support for net45, because it does not support IAsyncEnumerable