Proposal: Use MongoDB naming convention

[LantaoJin]

Description

This PR refactors the graphLookup command field names to align with MongoDB's $graphLookup aggregation stage naming convention, improving consistency with industry standards and making the API more intuitive for users familiar with MongoDB.

The original field names (startField, fromField, toField) were ambiguous and didn't clearly convey the relationship between fields. MongoDB's $graphLookup naming convention is
well-established and more descriptive:

• startWith - clearly indicates the initial value to start traversal with
• connectFromField - explicitly shows this is the field to recurse through
• connectToField - clearly indicates the field to match against

This alignment provides better developer experience and reduces confusion for users familiar with graph operations in MongoDB.

Field Name Mapping

Old Name	New Name	Description
startField	startWith	Field in source documents whose value starts the traversal
fromField	connectFromField	Field in lookup documents containing values to recurse through
toField	connectToField	Field in lookup documents to match against during traversal

Example Query

Before:

  source = employees
    | graphLookup employees
      startField=reportsTo
      fromField=reportsTo
      toField=name
      as reportingHierarchy

After:

  source = employees
    | graphLookup employees
      startWith=reportsTo
      connectFromField=reportsTo
      connectToField=name
      as reportingHierarchy

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[penghuo]

• I prefer option2 it is declarative syntax compare to mongodb's procedural grammar.

Option 1: Graph-native (align with Neo4j)
ppl
source = employees
| graphLookup employees
    start=reportsTo          -- Starting value
    via=name->reportsTo      -- Edge: match on name, traverse via reportsTo
    as reportingHierarchy


Option 2: Explicit (recommended)
ppl
source = employees
| graphLookup employees
    start=reportsTo          -- Initial value to match
    matchOn=name             -- Match incoming value against this field
    traverseVia=reportsTo    -- Get next value from this field
    as reportingHierarchy

[penghuo]

@LantaoJin Do we need this PR? Or Just merge to feature branch?

[LantaoJin]

How about this? @penghuo @model-collapse @qianheng-aws
before

source = employees
| graphLookup employees
    startField=reportsTo
    fromField=reportsTo
    toField=name
    as reportingHierarchy

after

source = employees
| graphLookup employees
    initialNode=reportsTo   -- graph-theory terminology for the starting node
    outboundField=reportsTo -- clearly signals the direction of traversal
    inboundField=name       -- counterpart to outboundField; receives the connection
    as reportingHierarchy

All terms are graph specific. It looks better then Option 2 (ref).

For Option 1, we implemented bidirectional traversal in our graph lookup. -> is directional only.