Add support to disable db while still setting the path

[stevehipwell]

Is your feature request related to a problem? Please describe.

I want to be able to only enable the DB functionality when I've configured writable storage on a host, as it currently stands I need to write custom logic into the Helm chart to remove the db config key is the host storage isn't writable.

Describe the solution you'd like

The db and related keys aren't currently YAML native so ideally a nested YAML format can be adopted along the lines of

env:
  STORAGE_ENABLED: "true"

pipeline:
  inputs:
    - name: tail   
      path: /var/log/containers/*.log
      db:
        enabled: ${STORAGE_ENABLED}
        path: /fluent-bit/data/tail.db

Describe alternatives you've considered

I remove the key in the Helm chart.

Additional context

The whole config pattern needs to be aligned to idiomatic YAML and at the same time the config should allow keys to be present and use guard flags (like enabled above) to control conditional logic.

[stevehipwell]

CC @patrick-stephens