[Feature] Use Unix Domain Socket (UDS) for sidecar from the etcd RPCs

[mvladev]

Feature (What you would like to be added):

Instead of running a TLS server for the initialization/status and initialization/start, use unix domain socket (UDS) to connect from etcd -> bacup sidecar. The socked can be created in a emptyVolume that is shared between containers.

I would very recommend to replace the bootstrap bash logic and replace with golang code.

Motivation (Why is this needed?):

This removes the need for TLS and unneeded overhead.

Approach/Hint to the implement solution (optional):

1. Create a new uds listener and add status and start endpoints to it.
2. Keep the TLS old code and HTTP handler for backwards compatibility.

[amshuman-kr]

Instead of running a TLS server for the initialization/status and initialization/start, use unix domain socket (UDS) to connect from etcd -> bacup sidecar. The socked can be created in a emptyVolume that is shared between containers.

Considering that there are other API which anyway are required (e.g. request on-demand full/delta snapshot) to be REST over TLS, I am not sure how much moving only this part to UDS would help.

I would very recommend to replace the bootstrap bash logic and replace with golang code.

Agreed. Earlier when we were using the upstream ETCD image, we were handicapped but now with gardener/etcd-custom-image we can pick this.