feat(GlFormInputGroup): Add `inputClass` prop to `GlFormInputGroup`

What does this MR do?

Adds an inputClass prop to GlFormInputGroup so CSS classes can be added to the form input.

Why is this needed?

I am working on a component that allows the user to show/hide and copy a secret token. Monospaced font is preferred for tokens so we want to add gl-font-monospace to the input.

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: gitlab!74771 (closed)
    • Will need to update a few snapshots specs
  • CustomersDot: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/4095
  • Status Page: GlFormInputGroup not used
• Added the ~"component:*" label(s) if applicable.