Convert project to monorepo

[gnarea]

We should eventually convert this repo to a monorepo to make it easier to maintain and manage its artefacts.

We could arrange the components based on the artefact they output, instead of having a single Docker image as the only output artefact:

• crc-cogrpc, the server for the CogRPC binding as a Docker image.
• pdc-pohttp, the server for the PoHTTP binding as a Docker image.
• crc-incoming-queue, the background job that processes incoming cargoes, as a Docker image.
• key-management, a script to generate/rotate/revoke identity (RSA) and session (ECDH) keys.
• etc...

We shouldn't be too granular to begin with to minimise complexity. For example, the two crc-* components could be one at the start.

Consider using https://rushjs.io/ to manage the monorepo.