Add ExpandableSection component

Addresses https://gitlab.com/gitlab-org/gitlab/-/issues/233521.

ee/app/assets/javascripts/security_configuration/sast/components/expandable_section.vue

+<script>
+import { GlButton, GlCollapse, GlCollapseToggleDirective } from '@gitlab/ui';
+import { uniqueId } from 'lodash';
+import { __ } from '~/locale';
+
+export default {
+  components: {
+    GlButton,
+    GlCollapse,
+  },
+  directives: {
+    CollapseToggle: GlCollapseToggleDirective,
+  },
+  props: {
+    headingTag: {
+      type: String,
+      required: false,
+      default: 'h3',
+    },
+  },
+  data: () => ({
+    collapseId: uniqueId('expandable-section-'),
+    visible: false,
+  }),
+  computed: {
+    toggleText() {
+      return this.visible ? __('Collapse') : __('Expand');
+    },
+  },
+};
+</script>
+
+<template>
+  <section
+    class="gl-py-6 gl-border-t-1 gl-border-b-1 gl-border-gray-100 gl-border-t-solid gl-border-b-solid"
+  >
+    <header class="gl-display-flex">
+      <div class="gl-flex-grow-1">
+        <component :is="headingTag" class="gl-font-size-h2 gl-mt-0" data-testid="heading">
+          <slot name="heading"></slot>
+        </component>
+        <p class="gl-mb-0" data-testid="subheading">
+          <slot name="subheading"></slot>
+        </p>
+      </div>
+
+      <gl-button
+        v-collapse-toggle="collapseId"
+        class="gl-flex-shrink-0 gl-align-self-start gl-ml-3"
+        >{{ toggleText }}</gl-button
+      >
+    </header>
+
+    <gl-collapse :id="collapseId" v-model="visible" data-testid="content">
+      <div class="gl-mt-6"><slot></slot></div>
+    </gl-collapse>
+  </section>
+</template>

ee/spec/frontend/security_configuration/sast/components/expandable_section_spec.js

+import { mount, shallowMount } from '@vue/test-utils';
+import ExpandableSection from 'ee/security_configuration/sast/components/expandable_section.vue';
+
+describe('ExpandableSection component', () => {
+  let wrapper;
+
+  const createComponent = (options, mountFn = shallowMount) => {
+    wrapper = mountFn(ExpandableSection, options);
+  };
+
+  const findButton = () => wrapper.find('button');
+  const findHeading = () => wrapper.find('[data-testid="heading"]');
+  const findSubHeading = () => wrapper.find('[data-testid="subheading"]');
+  const findContent = () => wrapper.find('[data-testid="content"]');
+
+  afterEach(() => {
+    wrapper.destroy();
+  });
+
+  describe('headingTag', () => {
+    it('defaults to h3', () => {
+      createComponent();
+
+      expect(findHeading().element.tagName).toBe('H3');
+    });
+
+    it('uses the given the heading tag name', () => {
+      const headingTag = 'h6';
+
+      createComponent({
+        propsData: { headingTag },
+      });
+
+      expect(findHeading().element.tagName).toBe(headingTag.toUpperCase());
+    });
+  });
+
+  describe('heading slot', () => {
+    beforeEach(() => {
+      createComponent({
+        slots: { heading: 'some heading' },
+      });
+    });
+
+    it('renders the given heading content', () => {
+      expect(findHeading().html()).toContain('some heading');
+    });
+  });
+
+  describe('subheading slot', () => {
+    beforeEach(() => {
+      createComponent({
+        slots: { subheading: 'some subheading' },
+      });
+    });
+
+    it('renders the given subheading content', () => {
+      expect(findSubHeading().html()).toContain('some subheading');
+    });
+  });
+
+  describe('default slot', () => {
+    beforeEach(() => {
+      createComponent({
+        slots: { default: 'some content' },
+      });
+    });
+
+    it('renders the given content', () => {
+      expect(findContent().html()).toContain('some content');
+    });
+  });
+
+  describe('expand/collapse behavior', () => {
+    beforeEach(() => {
+      createComponent({}, mount);
+    });
+
+    it('hides the content by default', () => {
+      expect(findContent().isVisible()).toBe(false);
+    });
+
+    it('shows an expand button', () => {
+      expect(findButton().text()).toBe('Expand');
+    });
+
+    describe('clicking the expand button', () => {
+      beforeEach(() => {
+        const button = findButton();
+        button.trigger('click');
+        return wrapper.vm.$nextTick();
+      });
+
+      it('shows the content', () => {
+        expect(findContent().isVisible()).toBe(true);
+      });
+
+      it('shows a collapse button', () => {
+        expect(findButton().text()).toBe('Collapse');
+      });
+    });
+  });
+});