Add app directory as a parameter

[anushkavidanage]

Pull Request Details

What issue does this PR address

• Add app directory as an optional parameter to the SolidLogin function.

• Link to associated issue: https://github.com/anusii/solidpod/issues/231

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
• [x] Changes adhere to the team style and coding guideline
• [x] No confidential information
• [x] No duplicated content
• [x] No lint check errors related to your changes (make prep or flutter analyze lib)
• [x] Pre-exisiting lint errors noted: [HERE]
• [x] Tested on at least one device
  • [ ] Android Phone
  • [ ] Android Emulator
  • [ ] Chrome on Android
  • [ ] Chrome
  • [ ] iOS
  • [ ] Linux
  • [ ] MacOS
  • [x] 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