GlFormInput - Add max length UI indicator

What/Why

Follow up from @dmishunov note here: gitlab!34561 (comment 367332934)

Based on design mocks that included a string max-length indicator:

Proposal

• Add an indicator to the input when type="text" and the prop max is provided.
• Count the indicator down based on the string length
• I don't think we should limit input after length max, just move to negative numbers and allow implementor to utilize their own form validations via GlFormGroup state prop.