Implement form input component for SAST Config UI

What does this MR do?

Add FormInput component

This adds a form input component, to be used in the SAST Configuration UI.

This component is a fairly thin wrapper around the GlFormGroup and GlFormInput components, to conform to the design. It will be consumed by a parent container component, itself driven via a schema to generate a form.

Future iterations will add more form components that follow this API. (Likely textarea and maybe dropdown are next.)

Addresses #225224 (closed).

Feature flag

While this component isn't used anywhere yet, it will be used in the SAST Configuration UI page, which itself is behind the sast_configuration_ui feature flag.

Screenshots

Note that this isn't actually used anywhere yet. But, I took these screenshots by just using them in the existing SAST Configuration Vue app.

Default value	Custom value

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done