fix & refactor: allow defining a custom cookie vault

[sehnryr]

Proposed changes

Allow defining a custom cookie vault with an optional HiveCipher as the encryption cipher when initializing Requests with init method.

cc @aramperes

Closes #62.

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)

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

Replaces #63 since its author did not allow edits by the maintainers.