Add documents upload form step

[aduth]

Why: As a user, I expect to be able to upload my ID documents, so that I can complete the identity verification step.

This is a basic initial implementation of the documents upload step as part of the new React-based document capture flow.

Screen Recording:

Implementation Notes:

• This makes use of the new File input component from USWDS, introduced in version 2.8.0.
  • Because identity-style-guide is currently running USWDS 2.0.3, it was implemented here by adding USWDS as a dependency and importing the stylesheet directly. It's unclear if this pull request should instead be blocked by a (pending) upgrade of USWDS in the Login Design System. The import should be relatively self-contained. In any case, long-term, it's not expected that uswds should need to be defined as a dependency of the project.
  • The File input component needs to be largely reimplemented due to current lack of support for localization in the default component implementation. See also Use localized data for interactive components uswds/uswds#3445 (comment).
• The FormSteps component was changed to support front and back being assigned in a single step, where now each step will receive the full current form values object, and is expected to trigger changes as a patch object.
• A new useInstanceId hook was introduced to simplify ID generation in accessible label association. This would be useful in the current Accordion component implementation, but I chose not to update it here, since it is pending removal at Update accordion to use Login Design System #3974, since it is not proposed to be used in current designs.

cc @anniehirshman-gsa

[zachmargolis]

LGTM

[aduth]

In some last-minute testing to see what potential side-effects could occur from this import, I noticed that the USWDS file input stylesheet applies some styles to file inputs globally. While the version of USWDS which is bundled with Login Design System does include some as well, they are not the same (margin, padding). This has the result that file inputs elsewhere in the application would have their styling altered unintentionally.

As a workaround, I updated the @import to be scoped to the #document-capture-form element used as the mounting point for the new React application, to ensure that styling only takes effect for the new page, until the design system can be upgraded as a whole.