What

This MR exposes parts of the Bounded_history_repr's structure via a view type & function

Why

We may need to access the content of the cache and store it in a more suitable way in the Smart rollup or DAL nodes.

By exposing the part of the type that's interesting (a map) via a view function

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).
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