dev: Refactor Adapty plugin to improve testability; Add linting; Upgrade versions constraints

[MichaelDark]

Changelog:

2.0.0

• BREAKING CHANGE: Adapty class is now singleton, that implements AdaptyPlugin contract (for future support for different platforms and testing).
• BREAKING CHANGE: To ease migration, Adapty class, that contained static methods, had been renamed to AdaptyLegacy.
• BREAKING CHANGE: Updated project structure. Use the following imports in dependent projects:

import 'package:adapty_flutter/adapty_flutter.dart';

// To avoid code changes in projects that are already using adapty_flutter,
// use the following import before migration:
import 'package:adapty_flutter/adapty_legacy.dart';

• BREAKING CHANGE: AdaptyProfileParameterBuilder does not have map getter anymore, it is replaced with build() method and returns new instance of map that is copy of internal _params field.
• BREAKING CHANGE: Change Dart version contraints: >=2.16.0 <3.0.0
• BREAKING CHANGE: Change Flutter version SDK contraints: >=2.10.0 <3.0.0
• Package is now been analyzed with lints package rules.
• Most error codes now have document comments.

[MichaelDark]

@x401om @iwitaly I need your help with the setup for example application!

[x401om]

Hello! Thank you for the awesome PR! We have recently released a new version of our SDK, in which we tried to take into account a bunch of your suggestions. Unfortunately, merging is not possible at the moment. If you suddenly see some things that can be improved, please make a new one, or create an issue.