Add the ability to attach merge strategies on objects as a Symbol

[dlespiau]

At the moment we have to explicitly tell merge the rules to merge two objects. For instance:

const pod = {
  spec: {
    containers: [{
      name: 'my-app',
      image: 'busybox',
      command: ['sh', '-c', 'echo Hello Kubernetes!'],
    },{
      name: 'sidecar',
      image: 'sidecar:v1',
    }],
  },
};

const sidecarImage = {
  spec: {
    containers: [{
      name: 'sidecar',
      image: 'sidecar:v2',
    }],
  },
};

merge(pod, sidecarImage, {
  spec: {
    containers: deepWithKey('name'),
  },
});

We could attach:

{
  spec: {
    containers: deepWithKey('name'),
  },
}

as a Symbol on the left-hand side object and have merge pick it up automatically (only if the 3rd argument to merge is undefined though).

We could start having well known jk.* symbols, say Symbol.for('jk.merge') for this one.