refactor(core): Implement RFC-2299 for stat_with

[Xuanwo]

part of #2300

from

if let Err(e) = op.stat_with("test", OpStat::new().with_if_match("<etag>")).await {
    if e.kind() == ErrorKind::NotFound {
        println!("file not exist")
    }
}

to

if let Err(e) = op.stat_with("test").if_match("<etag>").await {
    if e.kind() == ErrorKind::NotFound {
        println!("file not exist")
    }
}

[Xuanwo]

cc @suyanhanx to take another look.

[suyanhanx]

LGTM. Thanks.