Add if_not_exists in OpWrite to make behavior more clear

[Xuanwo]

OpenDAL now supports the use of "*" in if_none_match to perform conditional write operations:

let _ = op.write_with(path, bs).if_none_match("*").await?;

However, it's somewhat difficult to grasp at first glance. Do you think it's a good idea to include if_not_exists?

So we can write code like:

let _ = op.write_with(path, bs).if_not_exists(true).await?;

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After this change, we will still allow users to use if_none_match("*"), but we will no longer document it, making it a service implementation detail.