Migrate shared_preferences to stash_hive, dependencies upgrade and various fixes and improvements

[sehnryr]

Proposed changes

Migrated shared_preferences to stash_hive to not depend on flutter anymore, resolving issue #37.

Reverted the cast on line 409 in the request.dart file to fix an issue mentioned in #56.

Removed the author field from pubspec.yaml since it is deprecated.

Updated dependencies and migrated pedantic to lints as pedantic is deprecated.

Added description about 4.0.0-nullsafety.0 and this proposed 4.1.0 update.

Type of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

Further comments

Since hive uses AES 256 CBC with PKCS7 padding encryption, that could resolve issue #1.

As said in this comment, the authors field is deprecated and cannot be trusted since there is no validation for it. Therefore, it is not displayed anymore on pub.dev.