Add new api's to MtomMessageEncodingBindingElement

On .NET Framework, there is an app setting `"microsoft:xmldictionaryreader:maxmimeparts"` which controls the maximum number of mime parts allowed when using Mtom. The current implementation here uses the original hard coded limit of 1000. This needs to be configurable via a property on MtomMessageEncodingBindingElement.

Additionally, we need to provide a mechanism to pass a delegate which accepts a prefix, namespace, and localname and returns a value indicating whether an xml element should be inline or placed in a separate mime part, or to use the default decision. This will improve interop with other stacks as there have been issues in the past of other implementations not liking the decision made by .NET.