Enable password-like protection on sensitive fields like SSN

[bsradcliffe]

By default, browsers treat a password input with obfuscation in the form of character masking with circles. Likewise, sometimes users would enjoy similar protection on fields containing sensitive information like social security numbers. It'd be neat if Field Kit solutioned for this in a similar manner, with some kind of show/hide functionality in SSN fields.

[eventualbuddha]

This is available on the CardTextField. Something similar could be done for SSN, or TextField generally. Would be happy to review a PR 😄