Add Flutter toolchain for the Jason (#182)

[evdokimovs]

Part of #182

Synopsis

This PR adds basic toolchain for building Rust library for the Flutter.

Solution

• Add Flutter Plugin for Jason
• Add dummy Rust library which can be called from the Flutter side
• Add Rust library building for the iOS/Android platforms
• Add demo Flutter application

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains Draft: prefix
  • [x] Name contains issue reference
  • [x] Has k:: labels applied
  • [x] Has assignee
• [x] Documentation is updated (if required)
• [x] Tests are updated (if required)
• [x] Changes conform code style
• [x] CHANGELOG entry is added (if required)
• [x] FCM (final commit message) is posted
  • [x] and approved
• [x] Review is completed and changes are approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] Draft: prefix is removed
  • [x] All temporary labels are removed

[alexlapa]

FCM:

Bootstrap 'medea_jason' Flutter plugin (#190, #182)

- create 'medea_jason' Flutter plugin template
- setup Flutter toolchain in Makefile and on CI workflow