feat: start sketching out the oonirun package (#842)

This diff refactors the ./internal/cmd/miniooni pkg and moves the code
for running experiments inside of the ./internal/oonirun pkg.

It's the first concrete step towards https://github.com/ooni/probe/issues/2184.

internal/oonirun/doc.go

@@ -0,0 +1,5 @@
+// Package oonirun contains code to run OONI experiments.
+//
+// This package supports OONI Run v1 and v2 as well as the direct
+// creation and instantiation of OONI experiments.
+package oonirun