Ability to add api groups to existing client

Nobody has claimed this yet.

### overview of alternatives: https://github.com/abonas/kubeclient/issues/208#issuecomment-412706681

A variant of "union" client #208, #241:
Many people want one client object to combine methods from multiple API groups. The main questions are: union of *what* groups, *which version* per group, and who wins *method name collisions*?

This proposal is that Kubeclient would not try to guess. User would start from one group, and be able to add them one by one, each with specific version:
```
client = Kubeclient::Client.new('http://localhost:8080/', 'v1')
client.get_service_accounts

client.add_group('rbac.authorization.k8s.io/v1')
client.get_role_bindings
```
many people would add all desired groups when constructing client, and then just have the ability to call all methods:
```
client = Kubeclient::Client.new('http://localhost:8080/', 'v1')
client.add_group('rbac.authorization.k8s.io/v1')

client.get_service_accounts
client.get_role_bindings
```

Method name collisions would be decided by order of addition. Not sure if latest or earliest wins?

I think this can solve some concerns with #241.
It also can work better with CRDs that might appear dynamically, after client exists.

Start with Kubeclient::Client.new and review the alternatives in issues #208 and #241, along with the linked issue comment. Clarify group-version selection and method-collision precedence before defining the behavior for add_group; done means a client can combine explicitly selected API groups, including groups added after construction.

Written by the indexing model from the issue text.