OverrideFile API should add a param indicating whether to override by an equal sign or a colon

[zhuxw]

Enhancement

Currently, if a new config is added to a .k file using the OverrideFile API, the equal sign will be used. For example:

// Overriding prod/main.k:
// before override
container: Container {
}

// after override
container: Container {
   envs = {
        "FOO": "BAR"
   }
}

But sometimes this behavior is not expected, because the .k file is to be compiled together with other .k files. For example:

// base/base.k:  already exists some envs
container: Container {
    envs: {
         "aaa": "bbb"
    }
}

If prod/main.k is supposed to be compiled together with base/base.k, overriding using the equal sign will drop all the existing envs in base/base.k.

The expected behavior is that the OverrideFile API providing a new param, indicating whether to use equal sign or colon. For example:

overrideUsingColon := true
kclapi.OverrideFile(file, overrides, deps, overrideUsingColon)

[Peefy]

I think we can extend the spec https://www.kcl-lang.io/docs/user_docs/guides/automation#2-use-kcl-cli-for-automation with the : and += attribute operators.
cc @zong-zhe