Make filter label configurable in filter definition

[Flo0807]

Add the possibility to configure filter label in filter map / definition:

@impl Backpex.LiveResource
def filters do
  [
    category_id: %{
      module: MyAppWeb.Filters.Category,
      label: "Category"
    },
  ]
end

If no label is specified in the filter definition map, the label from the filter module is used. Therefore, this change is not breaking.