Enable VMR Codespace synchronization via the codeflow commands

[premun]

Context

The VMR has a codespace (devcontainer) definition, which has a script that allows users to synchronize code into the codespace from user forks: https://github.com/dotnet/dotnet/tree/main/.devcontainer

Based on the outcomes of https://github.com/dotnet/arcade-services/issues/4515 and https://github.com/dotnet/arcade-services/issues/4539, the way we synchronize code between repos and VMR will change (from darc vmr update to darc vmr back/forwardflow).

The synchronization script also needs to start using the new commands.

Goal

• Change the vmr-sync.sh script so that it uses the new codeflow commands.
  • The script is using src/sdk/eng/vmr-sync.sh which currently resides in the SDK repo but it is used for the current one-way synchronization. So this script should not be changed but rather maybe copied directly into the VMR (maybe under the .devcontainer dir) and edited there.
  • It needs to call darc vmr forwardflow from user's fork into the VMR folder in the codespace.