DAL: add Unpublished case to header_status type

(@iguerNL takes ownership while @julien.sagot.nl is off)

What

Add Unpublished case to header_status type.

Why

Distinguish data not found (e.g. slots in the future) and slots not published.

How

-type header_status = [`Waiting_attestation | `Attested | `Unattested]
+type header_status =
+  [`Waiting_attestation | `Attested | `Unattested | `Unpublished]

Manually testing the MR

Run a DAL node e.g. for a DAL node connecting to a ghostnet octez-node

./octez-dal-node run --data-dir $HOME/dal-node --history-mode=full --rpc-addr 127.0.0.1:10732 --net-addr 0.0.0.0:11732 --operator=0,1,2,3,4,5,6,7,8

Then, query some slot status and check the response given. Use https://explorus.io/dal in order to good levels / slots candidate for testing each case.

for l in 15686294 ; do for s in 00 01 02 03 04 05 06 07 08 09 10 11 12 13 ; do echo -n "[$s]" ; for p in 10732 ; do  echo -n " " $(curl "http://localhost:$p/levels/$l/slots/$s/status" 2> /dev/null) ; done ; echo ""; done ; done ;

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