service: allow mounting files

[boz]

Allow mounting files into a container.

Add a new section to the service section in sdl:

mounts:
  - path: /config/app.toml
    contents: |-
      foo=bar
      baz=boo

In k8s, create a configmap with the given contents and mount it at the given path.

To start, these can be mounted read-only.

[hydrogen18]

We will need to add limits on the size of the file that can be uploaded / injected when implementing this.