DAL/Node: add warning when `get_attestable_slots` is called for a future level

What

Add a warning when the get_attestable_slots is called for a level that suggests (if the baker made the call) that the DAL node is lagging.

Why

That's useful to know. And it's probably the only way to notice it automatically: IIUC, the crawler knows the current L1 level, and when it pushes a new block to the finalized stream, but it does not know when the daemon consumes a block from the stream.

How

Manually testing the MR

to be tested on Ghostnet

Checklist

• 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).