ci: use fastlane to automate version bumping, changelogs, git tags

[skywardpixel]

Issue #, if available:

Description of changes:

• Add fastlane config to automate version bumping, changelogs, git tags.
• Publishing to Pub.dev is still work in progress due to Google OAuth2 issues.
• This PR doesn't use Fastlane in CircleCI yet. You can test it locally by manually setting the env vars GITHUB_USER, GITHUB_EMAIL, CIRCLE_WORKING_DIRECTORY and merge it to master on your own fork.

export GITHUB_USER="fastlane-bot"
export GITHUB_EMAIL="[email protected]"
export CIRCLE_WORKING_DIRECTORY="~/path/to/amplify-flutter"

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jamesonwilliams]

Request reviews from @rjuliano @jpignata @iartemiev

[dnys1]

Thanks for the PR, @kaichengyan and sorry we were never able to merge this. We've adopted a new tool written in Dart to manage these tasks. All the best!