Support UDF function

[zhaojunnana]

We hope that Nebula graph can support functions similar to neo4j UDF, and support user-defined computing to push down to the nebula graph kernel.
We have a lot of aggregation and simple real-time computing needs online. If the database itself does not support the UDF function, a large number of returned results will be returned from graphd, so that the real-time performance can not meet the requirements.
Due to the limitation of current lexical analysis, it is difficult to support complete UDF function immediately. Consider parsing the keyword of UDF during lexical analysis, and then perform the calculation of UDF after the normal ngql statement is executed. It feels that the implementation difficulty may be less.

[awsl-dbq]

maybe we can follow the UDF in clickhouse , ClickHouse/ClickHouse#11
shoulders of giants

[arcosx]

Hi, a little later I will try to submit an issue about Wasm-based UDF, which I submitted at the Hackathon held by Nebula last year, and will put effort to improve it in the future.
The link is the idea at the time and may be modified in the future.
We can discuss with the Nebula developers.

[awsl-dbq]

excuse me , could you share the document of the link in github instead of docs.google.com.
A repo would be fine . so we can discuss it with issues or PRs.

[arcosx]

excuse me , could you share the document of the link in github instead of docs.google.com. A repo would be fine . so we can discuss it with issues or PRs.

Sorry for the delay, I've added some details inside this issue.#4060