Refactor session to follow wrapper class pattern

[angela-tran]

The Benefits app currently encapsulates interaction with the Django session framework via the benefits.core.session module, which contains:

• functions to retrieve session data
• a single update function to update session data
• some other helper functions.

This module serves as the single place where all session-handling logic is done, and we've noticed this is becoming harder to work with.

A new pattern was established in Office-of-Digital-Services/django-cdt-identity that follows a more class-based approach.

In the same spirit as #2778 and #2777, i.e. in the spirit of having smaller, more specific, self-contained and cohesive organization, we want to refactor the current session module into the new pattern. This could allow us to make use of inheritance or other language features to better organize the interaction with request.session. The Session in Office-of-Digital-Services/cdt-ods-disaster-recovery is an example of this.

Acceptance Criteria

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Additional context