Node: add helper to retrieve store from node

What

Small MR to expose access to the node.store.

Why

We will need it access to the store down the road.

Relative (head-n) queries are always changing which creates friction for integrating Octez RPCs with generic Caches as operators needs extra logic/heuristics to know when to invalidate such a response. To get around this, we introduce relevant HTTP Cache headers into the responses of Octez RPCs. The headers to be implemented are max-age and ETag with If-None-Modified. This is an ongoing work with multiple MRs.

2/n

1/n - !13326 (closed)

How

Adds a getter function in node.ml/i

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR