Support different options for loading token during cloud output run

[yorugac]

Cloud output is being added to k6-operator in #86 and this feature requires an authorization token to be loaded and passed to the pods. Currently, this is implemented with Kubernetes secret in operator's namespace; however, it would be good to support other options as well.

In particular, @brian-groux-hs suggested adding support for k6 config files and for secrets volumes of Hashicorp Vault.

Additionally, consider which logic is best to use for token loading: trying to load everything one by one or relying on a new configuration option.