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/oonimkall/sessioncontext.go

@@ -0,0 +1,29 @@
+package oonimkall
+
+import (
+	"context"
+	"math"
+	"time"
+)
+
+const maxTimeout = int64(time.Duration(math.MaxInt64) / time.Second)
+
+func clampTimeout(timeout, max int64) int64 {
+	if timeout > max {
+		timeout = max
+	}
+	return timeout
+}
+
+func newContext(timeout int64) (context.Context, context.CancelFunc) {
+	return newContextEx(timeout, maxTimeout)
+}
+
+func newContextEx(timeout, max int64) (context.Context, context.CancelFunc) {
+	if timeout > 0 {
+		timeout = clampTimeout(timeout, max)
+		return context.WithTimeout(
+			context.Background(), time.Duration(timeout)*time.Second)
+	}
+	return context.WithCancel(context.Background())
+}