[Feature][Kubectl-plugin] Make flags override yaml file for kubectl ray job submit

[MortalHappiness]

See #3627 (comment).

Expected behavior is that if filename and flags are both provided by the user, then take the filename as base to construct a RayJob object, and then override the values in that object with flags.

So if user provided a yaml file with headCPU set to 2 in it, and specifies the flag --head-cpu 3, then the resulting RayJob should set head CPU to 3.

[MortalHappiness]

@CheyuWu Do you want to work on this?

[CheyuWu]

Sure, thanks you