feat(GlButton): Add button-text-classes prop to gl-button

What does this MR do?

Adds a buttonTextClasses prop to gl-button to add additional classes to the inner gl-button-text span element. Multiple cases of gl-button that contain more than just text and require additional classes.

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.
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• Added the ~"component:*" label(s) if applicable.