Validate ProxiedApiRouteEndpointLookup provided configuration dictionary

[joaofbantunes]

The current implementation of ProxiedApiRouteEndpointLookup does not validate its configuration conveniently, as it only verifies that the provided dictionary is not null.

As it is a requirement, we should enforce that the dictionary keys correspond to a valid url path, composed of a single segment (e.g. /some-segment) and the values are valid url's.

With that in mind, a good approach is probably to start by changing the types contained in the dictionary, to something that better denotes the contents, then add some extra validations in the constructor.

1. Change the dictionary from Dictionary<string,string> to Dictionary<PathString,Uri>
2. In the constructor, check that the keys, are PathStrings that contain a single segment
3. In the constructor, check that the values are absolute uris (value.IsAbsoluteUri)
4. With all the information in hand regarding the validity of the keys and values, if any invalid entry is found, throw an ArgumentException, including information about what caused it