EPIC - gRPC CTA Frontend

Creating this issue to keep track of the workplan and ongoing work for the gRPC implementation of the CTA Frontend. Issues need to be tackled in the following order:

• #1065 (closed) - Implement the WFE calls and merge EOS-side changes
• #338 (closed) - Implement non-streaming admin commands
• #339 (closed) - Implement streaming admin commands
• #363 (closed) - Add systemtest