[User Management] Update GET users API to facilitate user search

[seanpreston]

Is your feature request related to a specific problem?

The frontend requires a way to search for users.

Describe the solution you'd like

• Update GET /api/v1/users/ to receive an optional query parameter username=<term>
• The endpoint should perform a query akin to FidesopsUser.query.filter(FidesopsUser.title.ilike('%<term>%'))

Additional context

Consider using https://sqlalchemy-utils.readthedocs.io/en/latest/orm_helpers.html#sqlalchemy_utils.functions.escape_like to prevent wildcards being passed in with the API input.