feat: add addCookie method

[sehnryr]

Proposed changes

Add addCookie method to add a cookie to a CookieJar without exposing cookie.dart.

Fixes #76.

Type of changes

What types of changes does your code introduce?

Put an x in the boxes that apply.

• 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)

Checklist

• I have read the CONTRIBUTING doc
• My code follows the Dart coding convention
• 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 commented my code, particularly in hard-to-understand areas
• I have added necessary documentation (if appropriate)

Further comments

Also fixed types that didn't get updated with 4.3.0 in the README while changing the example for the cookies.