feat: add placeholder, placeholderStyle to FormBuilderCupertinoTextField

[CyMathew]

Connection with issue(s)

Close #7

Solution description

Added optional placeholder and placeholderStyle to FormBuilderCupertinoTextField, so now users can add placeholder if they'd like

Screenshots or Videos

To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour Difficult to test placeholder text
• If apply, add documentation to code properties and package readme

[deandreamatias]

Thanks a lot for this contribution and sorry for delay merge. I lost the notification on all others.

[florisdipt]

@deandreamatias Can we get a new version release on pub.dev? Last one was from last year and its a shame to have development time put in to things that are not available there :)
Also because of #14 not being on pub.dev it is incompatible with newer flutter versions.

[deandreamatias]

@florisdipt Anyone can create PR with a commit (like this) to generate version and then I can push a tag. With this, automatically is pushed new version to pub.dev