common: Fetch schemas from Iglu Central before running tests

[istreeter]

The idea comes from this discussion.

Currently we have a lot of Iglu Central schemas hard-coded into the Enrich repo, e.g. here. We even have duplicates where different tests depend on the same schema.

It's a good that schemas are available locally when the tests run, because we do not want unit tests to have a dependency on a remote repository. But, it would be neater to fetch the schemas dynamically before the tests run instead of hard-coding them into this repo.

I suggest make a sbt task that fetches the schemas we need, and then put them in a resources directory which is accessible to all tests.