Implement Secret storage backend as gRPC server

[pkosiec]

Description

Changes proposed in this pull request:

• Add Protocol Buffers schema
• Generate gRPC client + server code from Protocol Buffers
• Implement gRPC server for secret storage backend
  • Support initially just AWS Secrets Manager. Tested also dotenv provider. The vault provider needs to be tested
  • Add tests for the gRPC server with fake secret provider

TODO

• [x] Update Development guide on website: https://github.com/capactio/website/pull/110
• [x] Create PR with dotenv and AWS Secrets Manager for teller repo (https://github.com/SpectralOps/teller/pull/60)

Notes

Lint failing because of timeout. Don't worry about it - The job will be green after review 🙂

Testing

To regenerate the gRPC code, run:

SKIP_DEPS_INSTALLATION=false make gen-grpc-resources

To run example calls against the gRPC service:

1. Create AWS security credentials with SecretsManagerReadWrite policy.

2. Export environment variables:

   export AWS_ACCESS_KEY_ID="{accessKey}"
   export AWS_SECRET_ACCESS_KEY="{secretKey}"

3. Run the server:

   APP_SUPPORTED_PROVIDERS=dotenv,aws_secretsmanager APP_LOGGER_DEV_MODE=true go run ./cmd/secret-storage-backend/main.go

4. Now let's use Go generated gRPC client to to see how the server behaves. Run the example pkg/hub/api/grpc/storage_backend/example_test.go.

   To run it, you can e.g. change the func ExampleNewStorageBackendClient() { to func TestE2EScenario(t *testing.T) { and run it as an usual test.

5. Change the line 12 of this test and run it again:

   provider := "aws_secretsmanager"

Related issue(s)

Resolves #637

[pkosiec]

Integration tests failed during Capact iunstallation, but the previous run was successful (https://github.com/capactio/capact/actions/runs/1900095690) - and also this PR doesn't change anything related to integration tests, so I'm merging this without retry.