[FEATURE] Support ExistsSubQuery PPL

[LantaoJin]

Is your feature request related to a problem?
Sub-task of #661

What solution would you like?

1. correlated condition:

SQL usage

SELECT  *
FROM    a
WHERE   EXISTS (SELECT  *
                FROM    b
                WHERE   b.id = a.id)

PPL usage

source=a | where exists [
  source=b | where b.id = a.id
]

2. uncorrelated condition:

SQL usage

SELECT  *
FROM    a
WHERE   EXISTS (SELECT  *
                FROM    b
                WHERE   b.id > 10)

PPL usage

source=a | where exists [
  source=b | where b.id > 10
]

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
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