Adds pre/post build hooks to service hooks definitions schema

[wbreza]

Pre/Post build hooks already exist but are missing from schema definition. This enables more discoverability for scenarios where users need to perform an action after restoring but before package.

One scenario is for the VITE integration for front-end web apps. Docker builds happen at the build state of the azd pipeline so users can hook into prebuild to set any VITE_ environment variables needed for building container images.