Support Optional `EdgeExpand` in GIE Runtime

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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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