GlSearchBoxByType: support setting custom class in input

In this MR gitlab-org/gitlab!52567 (merged), I needed to set the gl-search-box-by-type-input to 100%.

I think it will be helpful if we can apply our utility class via a prop.

<gl-search-box-by-type input-class="gl-w-full" />

Maybe there is another problem here 🤔 Not sure if this 100% width should just be applied on this component. Will need to investigate if anyone using this component has experienced a not full width when used with our GlDropdown component.

  <gl-form-input
    ref="input"
    :value="value"
    :disabled="disabled"
-   class="gl-search-box-by-type-input"
+   class="gl-search-box-by-type-input gl-w-full"
    v-bind="inputAttributes"
    v-on="inputListeners"
  />