Enable reload of modified models

[blecourt-private]

Suggestion for loading models from definitions instead of files which would be helpful in custom loaders.

Without reloading models after modifying them, custom loaders are more or less limited to using pure SQL in added or modified expressions.

This MR showcases a way to load models from their rendered definitions. The PR currently only adds support for sql models but I imagine support for python models and seed models can be added in a similar fashion.

I suppose support for python models, seed models plus a bunch of tests need to be added to make this PR proper but before proceeding with this I would appreciate some feedback. It wouldn't make sense to put more work into this if I'm completely on the wrong track.