Support injecting arbitrary Helm values

[markgoddard]

cofidectl makes it easy to use the SPIRE helm charts to deploy SPIRE, but does have some opinions on how they are configured. Other people have opinions too, and we should allow them to express them. It should be possible to inject arbitrary Helm values to be overlaid on those generated by cofidectl.

This would also make it easier for us to try out new SPIRE configurations without changing the cofidectl code.

Inspiration for this came from Kolla Ansible's custom service configuration feature, which allows OpenStack configuration to be flexibly customised using the native INI and YAML configuration formats. This avoids Kolla "owning the world", and allows users to refer to the documentation of the service being configured without needing to translate it into another format.