ESQL: Allow multiple columns to refer to the same channel/block

[costin]

Description

The layout builder does not allow for a channel to be associated with multiple attributes.
While this is possible, for implementation reasons, this did not trigger the issue.
Consider row a = 1| eval x = a, y = a
There's only one block a yet due to the way eval works, each attribute ends up creating a new channel which is backed by the same block (which has its own set of issue when a page is closed since the block cannot just be closed as it is being used in multiple places - see #100001).

Due to #100174, the issue is made more obvious since the eval x = a are converted into projections which do NOT copy the block but instead just point the new attribute to the same channel.
So row a = 1 | eval x = a, y = a becomes row a = 1 | project a, a as x, a as y where 3 different attributes refer to the same channel (0).
This is a much better representation of the situation and one that aligned with #100001.

Currently this check has been disabled as part of #100238 - I'm raising this issue to properly bubble the behavior up and potentially add more tests to validate the way forward.

[elasticsearchmachine]

Pinging @elastic/es-ql (Team:QL)

[elasticsearchmachine]

Pinging @elastic/elasticsearch-esql (:Query Languages/ES|QL)

[elasticsearchmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[costin]

This has been fixed as part of the block accounting work.