[ES|QL] Introduce rename expression AST nodes

[vadimkibana]

In the RENAME command we do not parse out the list of rename expressions:

RenameCommand = "RENAME" RenameExpression ("," RenameExpression)*

RenameExpression = Identifier "AS" Identifier

Which would have the AST:

{
  type: 'command',
  name: 'rename',
  args: [
    { type: 'rename' },
    { type: 'rename' },
    // ...
  ]
}

• Introduce the new RenameExpression AST nodes { type: 'rename' }.
• Make RENAME command produce a list of RenameExpression default arguments (not option arguments).

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See more detailed discussion here: #182393 (comment)

[elasticmachine]

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