Add missing Snowplow cookie attributes to the Subject class

[cpnat]

Is your feature request related to a problem? Please describe.
A common use case for the Python SDK is to add server side tracking to web applications written in Python, and relate this to tracking running client-side.

Since requests to server-side applications contain cookies, we are able to extract attributes from the Snowplow cookie (controlled by the javascript tracker client-side) which are relevant for stitching together sessions (namely the domain_sessionid and domain_sessionidx).

Describe the solution you'd like
Attributes and setter methods for domain_sessionid and domain_sessionidx are added to the Subject class.

This is a minimal change, and I can submit a PR if the suggestion is approved.

Describe alternatives you've considered
I am currently using a fork of the SDK to achieve this.

Additional context
N/A

[matus-tomlein]

Hi Colin Nattrass (@cpnat), thank you for the suggestion, this would be a welcome change! Please feel free to submit a PR for this.

[cpnat]

PR to add this functionality here -> #285