Migrate away from Vendoring of Meltano EDK

[joshua-janicas]

I was wondering if there is a good reason why dagster-ext is vendoring Meltano's EDK (https://github.com/quantile-development/dagster-ext/blob/master/meltano/edk/) rather than pinning a particular version from https://github.com/meltano/edk. With dagster-ext V0.1.2 to upgrade Pydantic (to allow later versions of 1.9 Dagster), it seems that was crashing on the annotations of missing :str on the description fields, which the Meltano EDK had already fixed.

For the purpose of future proofing the repo, I think we should be using Meltano's EDK as a package instead of including it directly in the folder structure.

[JulesHuisman]

Ah yes, the dagster-ext was build before the edk package existed / was stable. But this will need to be migrated to the pypi package in the future!