switch set() to add() for better inheritance.

[IronPan]

We will have a gcpOp that add some default volumes to the container, and we should allow upper level ops inheriting gcpOp to add more volumes, envs if they want.

A concrete example is having a op that read a GCS data and push to github. In this case I want to mount both GCP secret volume and Git secret volume.

TODO: we can add a batch method to add a list of resources, but that's not must have right now.

---

This change is 

[IronPan]

/assign @qimingj @Ark-kun

[Ark-kun]

/lgtm

P.S. In another PR, let's make all set/add operations chainable by adding return self. For future single-value chainable setters, a good naming is with_something.

[IronPan]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: IronPan

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• sdk/OWNERS [IronPan]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: IronPan

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• sdk/OWNERS [IronPan]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[IronPan]

/test presubmit-e2e-test