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/session.go

@@ -0,0 +1,33 @@
+package oonirun
+
+//
+// Definition of session.
+//
+// TODO(bassosimone): we should eventually have a common definition
+// of session (which probably means a few distinct definitions?) inside
+// the model package as an interface. Until we do that, which seems an
+// heavy refactoring right now, this local definition will do.
+//
+
+import (
+	"github.com/ooni/probe-cli/v3/internal/engine"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// Session is the definition of Session used by this package.
+type Session interface {
+	// A Session is also an InputLoaderSession.
+	engine.InputLoaderSession
+
+	// A Session is also a SubmitterSession.
+	engine.SubmitterSession
+
+	// DefaultHTTPClient returns the session's default HTTPClient.
+	DefaultHTTPClient() model.HTTPClient
+
+	// Logger returns the logger used by this Session.
+	Logger() model.Logger
+
+	// NewExperimentBuilder creates a new engine.ExperimentBuilder.
+	NewExperimentBuilder(name string) (engine.ExperimentBuilder, error)
+}