Extract base dropdown logic

What does this MR do and why?

This MR request is a part of bigger MR

Scope of this MR:

1. Create folder for shared dropdown components
2. Extract common logic to base_items_dropdown
3. Update group_projects_dropdown.vue to use new base dropdown
4. Add tests, update existing

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After
NO VISUAL CHANGES

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable feature flag:

   echo "Feature.enable(:policy_group_scope)" | rails c

2. Go to a group

3. Secure -> Policies

4. New policy => any type

5. Make sure there is an option all projects in linked groups

6. Make sure groups dropdown and projects dropdown work

Related to #480637 (closed)

ee/app/assets/javascripts/security_orchestration/components/policy_editor/pipeline_execution/action/code_block_file_path.vue

@@ -14,7 +14,7 @@ import { BV_SHOW_TOOLTIP, BV_HIDE_TOOLTIP } from '~/lib/utils/constants';
 import { getIdFromGraphQLId } from '~/graphql_shared/utils';
 import glFeatureFlagMixin from '~/vue_shared/mixins/gl_feature_flags_mixin';
 import RefSelector from '~/ref/components/ref_selector.vue';
-import GroupProjectsDropdown from 'ee/security_orchestration/components/group_projects_dropdown.vue';
+import GroupProjectsDropdown from 'ee/security_orchestration/components/shared/group_projects_dropdown.vue';
 import { isGroup } from 'ee/security_orchestration/components/utils';
 import { INJECT, OVERRIDE } from '../constants';
 import { validateStrategyValues } from './utils';