eip7732: swap beacon_block_root and slot in places in DataColumnSidecar (#4656)

This PR swaps `beacon_block_root` and `slot` in places in
`DataColumnSidecar`.
 
Motivation: There are currently two versions of `DataColumnSidecar`.
During deserialization from SSZ, it can be tricky to determine which
container version to deserialize into. Having the same byte offset for
the `slot` field in both versions allows clients to read the `slot`
value from the bytes (and thus gain insight into the container version)
before full deserialization.

Author: povi

specs/gloas/builder.md

@@ -172,8 +172,8 @@ def get_data_column_sidecars(
                 column=column_cells,
                 kzg_commitments=kzg_commitments,
                 kzg_proofs=column_proofs,
-                beacon_block_root=beacon_block_root,
                 slot=slot,
+                beacon_block_root=beacon_block_root,
             )
         )
     return sidecars

specs/gloas/p2p-interface.md

@@ -98,9 +98,9 @@ class DataColumnSidecar(Container):
     # [Modified in Gloas:EIP7732]
     # Removed `kzg_commitments_inclusion_proof`
     # [New in Gloas:EIP7732]
-    beacon_block_root: Root
-    # [New in Gloas:EIP7732]
     slot: Slot
+    # [New in Gloas:EIP7732]
+    beacon_block_root: Root
 ```
 
 ### Helpers