Adds 'member of' search types to user groups & remove erroneous extension attribute data_type validation

[w0de]

• "member of" is a valid search type for smart user groups, as with smart computer groups.

• The data_type field on an extension attribute accepts "String", "Integer", or "Date". As currently written, the validation for this field requires the value be either "String", "Integer", or an arbitrary date string matching the "Date" data_type's required format. We're not validating the actual ext attr's returned data here, ofc :), so can just delete this and use a standard string slice validation.

Here's an excerpted example API response for an attribute w/ data_type "Date":

{
    "id": 59,
    "name": "Last Munki Run",
    "enabled": true,
    "description": "",
    "data_type": "Date",