Add support for NebulaGraph

[Xuanwo]

Apply for this project: https://summer-ospp.ac.cn/org/prodetail/241190422

Notes

This project is hosted by @vesoft-inc, and opendal just provide mentorship.

Goal

Add nebula based service support so users can store data inside nebula via opendal.

The usage will look like:

let op = Operator::via_map(Scheme::Nebula, map)?;
let bs = op.read("path/to/file").await?;

Requirement

• Know about nebula's API
• Know about how to build/test rust project
• Better to have some knowledge about io-uring
• The current rust API for nebula's is incomplete; we may need to collaborate with upstream to finalize them.

Mentorship

Mentor: @suyanhanx
Discussion: mailing list and discord