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/.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,28 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug, triage
+assignees: bassosimone
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**System information (please complete the following information):**
+ - Device: [e.g. iPhone6]
+ - OS: [e.g. iOS8.1]
+ - Version [e.g. 22]
+
+**Additional context**
+Add any other context about the problem here.

internal/engine/.github/ISSUE_TEMPLATE/routine-sprint-releases.md

@@ -0,0 +1,29 @@
+---
+name: Routine sprint releases
+about: Bi-weekly releases of probe-engine, etc.
+title: ''
+labels: effort/M, priority/medium
+assignees: bassosimone
+
+---
+
+- [ ] psiphon: run ./update.bash
+- [ ] engine: update dependencies
+- [ ] engine: update internal/httpheader/useragent.go
+- [ ] engine: update version/version.go
+- [ ] engine: update resources/assets.go
+- [ ] engine: update bundled certs (using `go generate ./...`)
+- [ ] engine: make sure all workflows are green
+- [ ] engine: tag a new version
+- [ ] engine: update again version.go to be alpha
+- [ ] engine: create release at GitHub
+- [ ] engine: update mobile-staging branch to create oonimkall
+- [ ] cli: pin to latest engine
+- [ ] cli: update internal/version/version.go
+- [ ] cli: tag a new version
+- [ ] cli: update internal/version/version.go again to be alpha
+- [ ] android: pin to latest oonimkall
+- [ ] ios: pin to latest oonimkall
+- [ ] desktop: pin to latest cli
+- [ ] engine: create issue for next routine release
+- [ ] e2etesting: see whether we can remove legacy checks