Adding a proposal for existing hooks sidecar

Signed-off-by: Victor Toso <victortoso@redhat.com>

design-proposals/sidecar-hooks.md

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+# Sidecar hooks
+
+## Overview
+
+A hooking mechanism that allows customizations to the VMI before it starts without changing KubeVirt
+core components. The common user of those hooks are Sidecars that will run in the same Pod that the
+VMI will be deployed.
+
+## Motivation
+
+A way to accommodate use cases that we might not want to bring and support in KubeVirt itself or to
+give room to test and develop features while they are still not mature enough.
+
+## Goals
+
+- Provide and maintain a set of gRPC APIs for users to do modifications to the VMI
+- Provide and maintain sidecars that are important for core functionalities
+
+## Non Goals
+
+- Provide and maintain a wide variety of sidecars that are not part of KubeVirt's core
+  functionalities.
+
+## User Stories
+
+- As a VM owner, I want to apply custom configurations to QEMU command line
+  - For debugability, by adding support or tweak [to run with debug tools][]
+  - To test unsupported features or apply changes that [could workaround bugs][]
+- As a KubeVirt developer, I want to provide alternative solutions that might not be ready to core
+  KubeVirt components
+  - For Network configurations like [Slirp][] and [Passt][]
+
+## Design
+
+KubeVirt should provide a versioned gRPC Hook module, with all the APIs that can be used including
+parameters and responses.
+
+The gRPC server runs in the Sidecar. The Sidecars will use the module to establish the communication
+with virt-launcher and exchange capabilities to define which Hook version is being used and what are
+the hooks that are implemented.
+
+Virt-launcher will be entitled with managing the hooks. It'll verify and connect to all the Sidecars
+that are running by checking the pre-defined folder where the unix sockets for gRPC communication
+will be created.
+
+Virt-launcher should apply the requested changes to the VMI.
+
+## Hooks API promotion
+
+Should follow the same rules as stated in [API Graduation Guidelines][]
+
+[to run with debug tools]: https://kubevirt.io/user-guide/debug_virt_stack/launch-qemu-strace/
+[could workaround bugs]: https://github.com/kubevirt/kubevirt/issues/8420
+[Slirp]: https://github.com/kubevirt/kubevirt/pull/10272
+[Passt]: https://github.com/kubevirt/kubevirt/pull/10425
+[API Graduation Guidelines]: ../docs/api-graduation-guidelines.md