feat: Add validation state to CardField (#423)

[atiernan]

Closes #423

This adds three new fields to the card details returned by onCardChange: validCVC, validExpiryDate, and validNumber. On iOS, each field has a possible value of "Valid", "Invalid", "Incomplete", and "Unknown". The Android SDK doesn't seem to expose whether a field is incomplete or not, so Android has "Valid", "Invalid", and "Unknown".

This is my first time working on a native module so I'm not 100% on the Swift and Kotlin syntax, let me know if there's a better way to write this and I'll update it

[CLAassistant]

All committers have signed the CLA.

[arekkubaczkowski]

@atiernan great job but I am wondering if we should stay only with valid | invalid | unknown states to keep consistency on both platforms. @trag-stripe what do you think?

[atiernan]

For my use case the Incomplete state is a necessity, as my apps styling would be out of sync with the SDKs error styling (showing the error state when the field is empty/incomplete)

Would it not be better to have the ideal situation on iOS and push for an update to the Android SDK, to align them, rather than settle for a subpar solution on both platforms.

I can see the want for consistency between platforms though, so I'm happy to update the PR either way

[atiernan]

@csabol-stripe @michelleb-stripe Is there anything I can do to help progress this?

[giorgiofellipe]

Thanks @atiernan for the great work.

@arekkubaczkowski would be really great to have it merged and published.

Let me know if there is anything I can help.