Av/add onsubmit custom function

[anushkavidanage]

Pull Request Details

What issue does this PR address

• Add the ability to pass a onSubmit custom function to the widget builder. This function allows the user to have their own custom code to be run when the submit button is pressed

• Link to associated issue: #

Checklist

Complete the check-list below to ensure your branch is ready for PR.

Flutter Style Guide: https://survivor.togaware.com/gnulinux/flutter-style.html

• Screenshots included in linked issue
• Changes adhere to the team style and coding guideline
• No confidential information
• No duplicated content
• No lint check errors related to your changes (make prep or flutter analyze lib)
• Pre-exisiting lint errors noted: [HERE]
• Tested on at least one device
  • Android Phone
  • Android Emulator
  • Chrome on Android
  • Chrome
  • iOS
  • Linux
  • MacOS
  • Windows
• Added 2 reviewers (or 1 for private repositories then they add another)

Finalising

Once PR discussion is complete and 2 reviewers have approved:

• Merge dev into the branch
• Resolve any conflicts
• Add one line summary into CHANGELOG.md
• Bump appropriate version number in pubspec.yaml
• Push to git repository and review
• Merge PR into dev

[tonypioneer]

Looks good. Thanks @anushkavidanage.