Field: Add custom css classes

[pehbehbeh]

Idea from @krns:

Sometimes we need to override a select field only because of one additional class like w-36:

~H"""
    <div>
      <.form
        :let={f}
        for={%{}}
        as={:index_form}
        class="relative"
        phx-change="update-field"
        phx-submit="update-field"
        phx-target={@myself}
      >
        <%= Phoenix.HTML.Form.select(
          f,
          :index_input,
          @options,
          class: ["select w-36 select-sm", if(@valid, do: "hover:input-bordered", else: "select-error")],
          selected: @value,
          disabled: @readonly,
          id: @input_id,
          prompt: Map.get(@prompt, :prompt)
        ) %>
      </.form>
    </div>
    """

It would be helpful to pass additional classes to the fields.