Introduce an endpoint to update an org's `dbt_venv`

[fatchat]

To make it easier to upgrade dbt across clients, we want to allow the client to upgrade (and downgrade) their dbt version. The idea is that during an upgrade window we will have two versions available, and if changes are needed to the dbt code the client will be able to go back and forth between them. At the end of the upgrade window everyone will be forced to be on the new version

This issue asks for an endpoint to be introduced which the frontend would call to make this change. It should also create a table tracking available dbt versions along with their locations on disk i.e. the locations of their venvs