[Sessions] Split the directory `sessions/` into `sessions/core` and `session/http`

[LoicPoullain]

Issue

In the "session part", the HTTP stuff and non-HTTP stuff are a little mixed up. This causes two problems:

• the maintainability is harder due to coupling,
• it is harder to use sessions outside the HTTP hook such as in a GraphQL architecture.

Solution and steps

• [x] Split the components into two distinct directories.

Checklist

• [x] Add/update/check docs (code comments and docs/ folder).
• [x] Add/update/check tests.
• [x] Update/check the cli generators.