Add support for `SELF_HOSTED_LICENSE_KEY`

[0x2b3bfa0]

Currently it's an undocumented environment variable:

values.yaml

global:
  envVars:
    SELF_HOSTED_LICENSE_KEY: ...

Let's consider documenting it as part of the values structure, and storing the value in a Kubernetes secret:

values.yaml

global:
  license: ...

Similarly, we should explore the option of unifying this setting and the container image pull secret, because in practice they're already the same.