Make it easier to bring your own binary by supporting toolchain_utils pattern

[mikn]

I notice that the toolchains for helm and kustomize are implemented using custom (and private) providers, and the executables are referred through these providers;

    kustomize_tool = ctx.toolchains[_kustomize_toolchain_type].kustomizeinfo.tool
    helm_tool = None
    args = ctx.actions.args()

    args.add(_runfile_path(ctx, kustomization.target_file))
    if kustomization.requires_helm:
        info = ctx.toolchains[_helm_toolchain_type].helminfo
        if not info:
            fail("No Helm toolchain is available; unable to proceed with invoking kustomize without it")
        helm_tool = info.tool

We were planning on using the excellent toolchain_utils that ARM is publishing to wrap our binary management system, but since the toolchain implementation is not using the executable field on platform_common.ToolchainInfo, we cannot do that.

Would you be open to using the executable field on the ToolchainInfo provider instead to pass the path to the rules from the toolchain to make it easier to BYOB (Bring Your Own Binary)?
Ref: https://gitlab.arm.com/bazel/toolchain_utils/-/blob/main/toolchain/info/rule.bzl?ref_type=heads