[BUG] _id is not available in PPL where condition

[LantaoJin]

What is the bug?
#639 reported the issue in SQL and addressed by #1456. But the problem is still existed in PPL.

How can one reproduce the bug?

POST _plugins/_ppl
{
"query": "source=my-index1 | where _id = 1"
}

SemanticCheckException was encountered

What is the expected behavior?
Allow metadata fields in PPL query, as same as SQL query.

What is your host/environment?

• Version 2.15
• Plugins SQL

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Do you have any additional context?
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