Moving `if_not_exists` out of meta request body.

[ariesdevil]

          I suggest moving `if_not_exists` out of this struct and let the caller determine what to do.

if_not_exists only affects the result returned to the user, not the behavior of the meta-service operation. Therefore, it would be better if SchemaApi returns either a Created or an Exist status code, and lets the caller decide whether to emit an error or not.

Originally posted by @drmingdrmer in https://github.com/datafuselabs/databend/pull/11414#discussion_r1192378396