Add cookie validator

[emilejq]

Proposed changes

Rules for cookies are currently only able to be applied to all cookies. This PR adds a cookie validator so that rules can be applied on a per-cookie basis. For example:

set-cookie:
    required: True
    cookies:
        my-cookie:
            required: True
            must-contain: SameSite=Strict

As the validation methods were getting a bit long doing header, directive and cookie validations, I've created separate validator classes for each

Type of change

What types of changes do you want to introduce to DrHeader?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change
• Documentation Update
• Test Update
• Rules Update
• Other (please describe)

Please ensure your pull request adheres to the following guidelines:

• A Github Issue that explains the work.
• The changes are in a branch that is reasonably up to date
• Tests are provided to reasonably cover new or altered functionality.
• Documentation is provided for the new or altered functionality.
• You have the legal right to give us this code.
• You have adhered to the CoC

Link to the github issue

#228

Tests you have added

test_validators.py

test_drheader.py:
test_cookie__exists_validation_ko
test_cookie__not_exists_validation_ko
test_cookie__must_avoid_validation_ko
test_cookie__must_contain_validation_ko
test_cookie__must_contain_one_validation_ko

Anything Else

Thanks for getting this far !