Add support for the JSON.ARRAPPEND command

[PragmaTwice]

Please refer to #1805.

[lieck]

I want to take this issue, thanks.

[git-hulk]

@lieck Assigned, thank you!

[mapleFU]

Oh I found a interesting thing in RedisJson...

///
/// JSON.ARRAPPEND <key> <path> <json> [json ...]
///
pub fn json_arr_append<M: Manager>(
    manager: M,
    ctx: &Context,
    args: Vec<RedisString>,
) -> RedisResult;

The code of it is JSON.ARRAPPEND <key> <path> <json>. But the document of it says the path is optional, it's so misleading...

Our current impl is right