Support for affinity

# Support for affinity

Add support for `affinity` injection.

This will include the configuration format for the node selectors.

Out of the three main things we will modify, affinity is probably the hardest to implement as we have `nodeAffinity`,
`podAffinity` and `podAntiAffinity`, all of which our user could configure. Consult the [Kubernetes Reference][1] for
more detail.

All the affinity types use node selector terms, either in pure form or array form. As such, we can probably reuse the
validation code for the `nodeSelector` and `toleration` features, and they should be implemented earlier.

TODO: List all the possible cases we need to test for.

As a suggestion, we can probably ignore `podAffinity` and `podAntiAffinity` for the time being, as the use cases for "making pods in this namespace run in the same/different nodes than these other pods" are waaay less frequent/likely than our simple initial case of "making pods in this namespace run in these nodes".

[1]: https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.28/#affinity-v1-core