Remote integration tests

Testing against real-world, remote services, as opposed to local mock services, is something that keeps popping up in discussions:

1. https://github.com/timberio/vector/pull/2412#issuecomment-620284867
2. https://github.com/timberio/vector/pull/2222/files#diff-8c0f62c7b5cebce3dc01858e75b09369R1017
3. https://github.com/timberio/vector/issues/2427

The hesitation centers around adding another test resource that we have to manage and maintain. I think we can reduce the burden if we:

1. Limit remote integrations tests as much as possible. Keep them simple (it works or it doesn't) and lean into local unit tests for everything else.
2. Have these remote test resources run 24/7 so we do not have to orchestrate provisioning and teardown. My hope is that this will make things simpler.
3. Add a tests/terraform folder that makes it easy for anyone to provision these resources.

I'm curious what others think.

Review the linked discussions and the proposal for a tests/terraform folder first. The issue needs a decision on which remote services to support, how they should run continuously, and what provisioning workflow is expected before implementation can begin.