[ES|QL] Nested quoted column identifer parsing

[vadimkibana]

ES|QL grammar allows to quote each part of a column identifier separately, for example below kept column:

FROM index | KEEP `a`.`b`

is equivalent to simply a.b unquoted column:

FROM index | KEEP a.b

But is not equivalent to:

FROM index | KEEP `a.b`

What needs to be done:

• Bug: ES|QL parser currently keeps the inner backticks as part of the column name: a`.`b.
• Parse out a list of identifiers.

---

In our column node type, we need to introduce a list of column identifiers:

{
  type: 'column',
  parts: ['a', 'b'],
}

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)

[stratoula]

Removing the Discovery team as this is on the ESQL team codebase