refactor the options for nebula graph

[HarrisChu]

the initial purpose for this k6 plugin was just testing the performance for NebulaGraph.

there're some more options for NebulaGraph, e.g.

HTTP2
security with SSL
SessionPool
Retry policy
...

And it's more complex if keep the original behavior.

So I want to break the original behavior, and using an option like k6, i.e.

let poolOption = {
    key : value
}
pool.setOption(poolOption)

And then document the valid options for nebula graph pool