Allow to decide which operator to use when filtering a string attribute

[roggervalf]

When an attribute is an string and you want to filter using an uuid that is supposed to be contained in this attribute, https://github.com/SoftwareBrothers/adminjs-sequelizejs/blob/master/src/utils/convert-filter.ts#L18 it uses an exact match, instead of the contains operator https://github.com/SoftwareBrothers/adminjs-sequelizejs/blob/master/src/utils/convert-filter.ts#L38