Extract independent modules to dedicated repos

[nulls]

Most developments in save-cloud happens in save-backend and save-frontend, but we build all modules in scope of PR.

We can extract to dedicated repos:

• save-api
• save-agent and save-demo-agent
• api-gateway

Props:

1. It should speed up the build (but not significantly, only save-agent and save-demo-agent is blocker for a build and they take ~4 minutes (each of them))
2. We create a release for save-api because it's provided as a library for save-backend-tests