LG-5247: Add form input hint styles

[aduth]

Why: To increase the contrast between the label and hint.

Related Slack discussion: https://gsa-tts.slack.com/archives/C01JVKYE4KD/p1634220766001700

Live preview URL: https://federalist-2f194a10-945e-4413-be01-46ca6dae5358.app.cloud.gov/preview/18f/identity-style-guide/aduth-hint-italic/components/form-fields/#format-constraints

Screenshot:

[nickttng]

Looks good to me.

Curiosity question: Here, the example is "000-123-4567" while the implemented example is "123-45-6789." Any reasons why they differ or if there is a case to use "000" digits over the latter?

[jmdembe]

LGTM

[aduth]

Curiosity question: Here, the example is "000-123-4567" while the implemented example is "123-45-6789." Any reasons why they differ or if there is a case to use "000" digits over the latter?

Ah, to be honest, I hadn't referenced the design for the specific text of the example. I chose the number because according to an SSA fact sheet I found via Google, a valid SSN would never start with 000.