Implement Helm release storage backend

[mszostok]

Description

Changes proposed in this pull request:

• Implement Helm release storage backend

Related PR: capactio/hub-manifests#62. In backend, we have an assumption that the data is validated by Local Hub, so we receive a valid context with all required fields.

Testing

Setup

1. Create k8s cluster. For example, run:

   k3d cluster create test

2. Install PostgreSQL Helm chart:

   RUNNER_CONTEXT_PATH=cmd/helm-runner/example-input/context.yaml \
    RUNNER_ARGS_PATH=cmd/helm-runner/example-input/install-args.yaml \
    RUNNER_LOGGER_DEV_MODE=true \
    RUNNER_COMMAND="install" \
    go run cmd/helm-runner/main.go

Testing

1. Run server:

   APP_LOGGER_DEV_MODE=true APP_MODE="release" go run ./cmd/helm-storage-backend/main.go

2. Run showcase test:

   GRPC_SECRET_STORAGE_BACKEND_ADDR=":50051" go test ./internal/helm-storage-backend/... -run "^TestShowcase$" -v -count 1

There is also unit test coverage.

Related issue(s)

Fix: #671

[pkosiec]

Works well 👌 A few minor comments and this PR is ready to be merged 🙂