Update component code adding description for form input fields

In gitlab-org/gitlab-services/design.gitlab.com#642 (closed), we added an optional description area below the title of an input field. This issue is to update the component code.

Former text input limitations	Proposed change - adds description field under label
Descriptions could only go below the text input ("help text")	Adds a description field under the label (doesn't limit designers to using the help text below the input field for a description

Implementation Plan

• In gitlab-ui add a labelDescription and label prop to https://gitlab.com/gitlab-org/gitlab-ui/-/blob/main/src/components/base/form/form_group/form_group.vue

• Add label slot to https://gitlab.com/gitlab-org/gitlab-ui/-/blob/main/src/components/base/form/form_group/form_group.vue so end up with something like

• Update story book examples + add examples

• Update unit tests and manually test in gitlab codebase.

<template>
  <b-form-group v-bind="$attrs" class="gl-form-group" :label-class="actualLabelClass">
    <template #label>
      <template v-if="label">{{ label }}</template>
      <gl-form-text v-if="labelDescription" class="gl-mt-3">{{ labelDescription }}</gl-form-text>
    </template>

    <slot v-for="slot in Object.keys($slots)" :slot="slot" :name="slot"></slot>
  </b-form-group>
</template>