[Tables] TableEntity metadata should only be accessible by an attribute

[seankane-msft]

Currently metadata can be accessed by either entity['_metadata'] or entity._metadata. We want to remove both of these options and leave the entity.metadata() as the only way to access this attribute.

[seankane-msft]

Note: adding

    def __getattr__(self, name):
        """
        :param name:name of entity entry
        :type name: str
        :return: TableEntity dictionary
        :rtype: dict[str,str]
        """
        if name == '_metadata':
            raise AttributeError(_ERROR_ATTRIBUTE_MISSING.format("TableEntity", name))
    ...

to the TableEntity object makes the entity.metadata() function no longer work.