feat(GlFormGroup): let label class be customized

What does this MR do?

It is currently impossible to pass custom label classes to BFormGroup via the labelClass prop as it being overridden in the GlFormGroup wrapper: https://gitlab.com/gitlab-org/gitlab-ui/-/blob/96b8f0189c3fa6ca9c9ec28afe525e922fbf9cc7/src/components/base/form/form_group/form_group.vue#L12

This MR fixes this by re-exposing the labelClass prop in the wrapper, and computing BFormGroup's prop by appending our default class to it.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.