Notify developers when devcontainer is changed to rebuild workspace

[bpmct]

We want to add notifications into our product. We should wait until this is complete before announcing v1.0 and leverage it to notify users when the devcontainer changed and prompt them to rebuild their workspace.

[johnstcn]

This may need an RFC to flesh out exactly how we want to handle the envbuilder -> coder integration, given that we want to avoid direct integrations when possible.

[bpmct]

yeah, let's pause on this until we have generic notifications support