chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/submitter.go

@@ -0,0 +1,67 @@
+package engine
+
+import (
+	"context"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// TODO(bassosimone): maybe keep track of which measurements
+// could not be submitted by a specific submitter?
+
+// Submitter submits a measurement to the OONI collector.
+type Submitter interface {
+	// Submit submits the measurement and updates its
+	// report ID field in case of success.
+	Submit(ctx context.Context, m *model.Measurement) error
+}
+
+// SubmitterSession is the Submitter's view of the Session.
+type SubmitterSession interface {
+	// NewSubmitter creates a new probeservices Submitter.
+	NewSubmitter(ctx context.Context) (Submitter, error)
+}
+
+// SubmitterConfig contains settings for NewSubmitter.
+type SubmitterConfig struct {
+	// Enabled is true if measurement submission is enabled.
+	Enabled bool
+
+	// Session is the current session.
+	Session SubmitterSession
+
+	// Logger is the logger to be used.
+	Logger model.Logger
+}
+
+// NewSubmitter creates a new submitter instance. Depending on
+// whether submission is enabled or not, the returned submitter
+// instance migh just be a stub implementation.
+func NewSubmitter(ctx context.Context, config SubmitterConfig) (Submitter, error) {
+	if !config.Enabled {
+		return stubSubmitter{}, nil
+	}
+	subm, err := config.Session.NewSubmitter(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return realSubmitter{subm: subm, logger: config.Logger}, nil
+}
+
+type stubSubmitter struct{}
+
+func (stubSubmitter) Submit(ctx context.Context, m *model.Measurement) error {
+	return nil
+}
+
+var _ Submitter = stubSubmitter{}
+
+type realSubmitter struct {
+	subm   Submitter
+	logger model.Logger
+}
+
+func (rs realSubmitter) Submit(ctx context.Context, m *model.Measurement) error {
+	rs.logger.Info("submitting measurement to OONI collector; please be patient...")
+	return rs.subm.Submit(ctx, m)
+}