[Feature] Support graphlookup as a top-level PPL command

[qianheng-aws]

Is your feature request related to a problem?
Currently, graphlookup can only be used as a piped command following another source-providing command (e.g., source=... | graphlookup ...), where startWith is resolved from the piped input.

This proposal adds support for graphlookup as a top-level PPL command (i.e., as the first command in a query without requiring source=). In this mode, the startWith option accepts either a single literal value or a list of literal values to seed the graph traversal. It will get simpler on syntax and also more execution efficiency for there common graph search cases where the start value is already know as a literal.

Current syntax (pipe-only)

source = employees | graphLookup employees start=reportsTo edge=reportsTo-->name as reportingHierarchy

Proposed extended syntax (top-level, no source required)

graphLookup employees start="Jack" edge=reportsTo-->name as reportingHierarchy
graphLookup employees start=("Jack", "Eliot") edge=reportsTo-->name as reportingHierarchy

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[srikanthpadakanti]

Hey @qianheng-aws Please assign this to me.

[qianheng-aws]

Hey @qianheng-aws Please assign this to me.

@srikanthpadakanti I think it's an good-for-agent task and am trying to solve it with Cloud Code locally. Let's see if it works well.