Fetch group labels after group selection

What does this MR do?

Currently we fetch the group labels when the custom stage form is loaded, now that we have a group selector, we should fetch the labels after the group selection has been made, ensuring the group label data available in the vuex store for #33604 (closed) and #13076 (closed)

This MR:

• Adds actions to fetch group labels and update the store
• Removes the custom_stage_form_data actions
• Adds a showCustomStageForm action to display the custom stage form

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team