Consider adopting containerd's shim bootstrap protocol

[mxpv]

In containerd 2.3 we've introduced a new way to launch shims - Shim Bootstrap Protocol.

TLDR: Starting with containerd 2.3, the start command receives all configuration from containerd via a single protobuf-serialized BootstrapParams message on stdin. This replaces the previous scattered mechanisms (CLI flags, environment variables, stdin protobuf options) with a single, versioned, extensible protocol.

• Docs: https://github.com/containerd/containerd/blob/main/docs/runtime-v2.md#bootstrap-protocol-23
• Introduce shim bootstrap protocol containerd/containerd#12786

However, the new path is currently disabled for shim-runhcs-v1. hcsshim hard-codes the assumption that stdin is a marshalled with runhcsopts.Options (which we now pass as an extension) and hard fails.

The call chain:

• hcsshim/cmd/containerd-shim-runhcs-v1/start.go

  Line 149 in 06fce47

  Stdin: os.Stdin,

• hcsshim/cmd/containerd-shim-runhcs-v1/serve.go

  Line 75 in 06fce47

  shimOpts := &runhcsopts.Options{

It'd be great to have hcsshim's readOptions (serve.go) to first attempt to decode the input as BootstrapParams and extract the Options from its extension (AddExtension/GetExtension), falling back to the legacy Any decode for older containerd.

Relevant runc v2 example:

func (manager) Start(ctx context.Context, opts *bootapi.BootstrapParams) (_ *bootapi.BootstrapResult, retErr error) {
...
    var runcOpts options.Options
    if found, err := opts.FindExtension(&runcOpts); err != nil {
        return nil, fmt.Errorf("failed to fetch runc options: %w", err)
    } else if found {
        if shimCgroup := runcOpts.GetShimCgroup(); shimCgroup != "" {
            ... // join the shim cgroup
        }
    }