[FEATURE] Support running sidecar without object store

[ttousai]

Is your feature request related to a problem? Please describe.
I want to run the Thanos sidecar without specifying an object store.
This mode is supported in the Thanos documentation
but it is not supported in puppet-thanos.

Describe the solution you'd like
Ability to start the sidecar without specifying the --objstore.config-file or puppet-thanos setting a default value.

Describe alternatives you've considered
None.

Additional context
None.