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/probeservices/login.go

@@ -0,0 +1,38 @@
+package probeservices
+
+import (
+	"context"
+	"time"
+)
+
+// LoginCredentials contains the login credentials
+type LoginCredentials struct {
+	ClientID string `json:"username"`
+	Password string `json:"password"`
+}
+
+// LoginAuth contains authentication info
+type LoginAuth struct {
+	Expire time.Time `json:"expire"`
+	Token  string    `json:"token"`
+}
+
+// MaybeLogin performs login if necessary
+func (c Client) MaybeLogin(ctx context.Context) error {
+	state := c.StateFile.Get()
+	if state.Auth() != nil {
+		return nil // we're already good
+	}
+	creds := state.Credentials()
+	if creds == nil {
+		return ErrNotRegistered
+	}
+	c.LoginCalls.Add(1)
+	var auth LoginAuth
+	if err := c.Client.PostJSON(ctx, "/api/v1/login", *creds, &auth); err != nil {
+		return err
+	}
+	state.Expire = auth.Expire
+	state.Token = auth.Token
+	return c.StateFile.Set(state)
+}