Support Optional `EdgeExpand` in GIE Runtime

[BingqingLyu]

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As titled. Support an optional edge expand, which is used to further support optional match in queries. Basically, given a vertex $v$, optional edge expand is to expand its neighbors if any, or represents the absence of neighbors with None, while preserving such $v$ in the results.

This includes:

• [x] Add a field of is_optional in EdgeExpand in proto to express optional edge expand
• [x] Support a basic implementation of OptionalEdgeExpand
• [ ] Support more complex situations like OptionalEdgeExpand in ExpandIntersection and PathExpand

Besides, this will introduce Null value, we need to process it appropriately in other operations. E.g., expanding from a Null value may also leads to a Null result.

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[github-actions[bot]]

/cc @BingqingLyu, this issus/pr has had no activity for for a long time, could you folks help to review the status ? To suppress further notifications,

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