Filters contacts in an organization using Whippy Query Language (WQL).

Use this endpoint for complex audience-style filters across contacts and related resources
(messages, conversations, tags, segments, custom data, and more).

Contact dates of birth are stored on the embedded birth_date object and are queried
through the birth_date.day, birth_date.month, and birth_date.year columns.

When a filter item includes cast, its value must be a string — or, for list
comparisons such as in, not_in, and between, a list of strings, each validated
with the same rules. Each string is an ISO date/datetime, or (with
"from": "relative_datetime") a relative datetime expression such as
"+0d Europe/Dublin" — an offset (+0d is today, -1d is yesterday) followed by an
IANA timezone. Casting a relative datetime to day_number/month_number compares
against the day/month of the resolved date. For example, to find contacts whose
birthday is today:

{"filter": [{"resource": "contact", "column": "birth_date.day", "comparison": "==",
  "value": "+0d Europe/Dublin", "cast": {"from": "relative_datetime", "to": "day_number"},
  "and": [{"resource": "contact", "column": "birth_date.month", "comparison": "==",
    "value": "+0d Europe/Dublin", "cast": {"from": "relative_datetime", "to": "month_number"}}]}]}

Returns matching contacts with a total count. Supports limit (default 50, max 500),
offset, and sort.