What

This MR adds some fixes and exports in the protocol regarding Dal to ease indexing the cells of the (Dal) skip list

Why

The Dal skip list cells are necessary to build refutation Dal involving Dal, and having a simple way to index them is important.

We expose an internal function that allows to compute attested slots for a list of published slots and a function that returns whether a slot is attested
Function add_cell (internal helper of add_confirmed_slot_headers) is modified to systematically add all the cells of a given level in the cache, instead of having an off-by one (that was adding the last cell of the previous level and postpones the addition of the last cell to the next level)

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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