DNN-5015 Mark ExtensionUrlProviderInfo as Serializable

Types which inherit from ExtensionUrlProvider cannot be serialized, since the
base type has a property of type ExtensionUrlProviderInfo, which is not marked
with the Serializable attribute
Merge pull request #90 from bdukes/DNN-5015

DNN-5015 Mark ExtensionUrlProviderInfo as Serializable