Build out monodeploy stages using plugin system

[noahnu]

Is your feature request related to a problem? Please describe.

Similar to Webpack, take advantage of the plugin system to build out the internal monodeploy stages/pipeline. We'd have "defaults" / "core" and then allow developers to extend the pipeline and alter behaviour using plugins.

Describe the solution you'd like

Rewrite core pipeline stages using the plugin system we're already using for plugin-github.