validate operators for join and custom fields

[Mathias Polligkeit]

The field type of normal schema fields can be determined with the Ecto schema reflection functions. For join and custom fields, this is not possible, which is why currently, all operators are accepted for those fields.

@derive {
  Flop.Schema,
  join_fields: [pet_species: [binding: :pets, field: :species, ecto_type: :string]],
  custom_fields: [
    inserted_at_date: [filter: {CustomFilters, :date_filter, [source: :inserted_at]}, ecto_type: :date]
  ]
}

• add ecto_type option to join fields
• add ecto_type option to custom fields
• determine Flop field type in Flop.Filter.allowed_operators/2 and retrieve Ecto field type from option, if applicable