Implement Go Template storage backend service

[mszostok]

Description

In Capact manifests, Content Developer is able to render config based on other TypeInstances data, see:
https://github.com/capactio/hub-manifests/blob/9987550e2341cc761c8613cc4e6bb716d5a328f1/manifests/implementation/gcp/cloudsql/postgresql/install-0.2.0.yaml#L102-L108

This is not supported directly in new external backends. We want to have a dedicated backend which will do this projection.

Read more on Delegated TypeInstance projection in delegated proposal.

AC

• Dedicated backend storage for TypeInstance projection created under cmd/ in Capact repo.
• Support Go templates
• Backend dedicated documentation under https://capact.io/docs/next/feature/storage-backends/introduction

Related issues

See epic #604 for reason and use cases.