Adds support for reading custom attributes

[irvinesunday]

Fixes #192

This PR:

• Provides a means of reading custom attribute values from the CSDL through the setting CustomXMLAttributesMapping, which a client can use to provide a dictionary mapping of custom attribute names and extension names. The values of these custom attribute names will be extracted from the Edm model from an element if it contains the custom attribute. The values will then be mapped to the custom extension values provided in the dictionary and added to the relevant extensions object in the OpenAPI document.
• Tests have been added (and integration files updated) to validate the above.