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/statefile.go

@@ -0,0 +1,87 @@
+package probeservices
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// State is the state stored inside the state file
+type State struct {
+	ClientID string
+	Expire   time.Time
+	Password string
+	Token    string
+}
+
+// Auth returns an authentication structure, if possible, otherwise
+// it returns nil, meaning that you should login again.
+func (s State) Auth() *LoginAuth {
+	if s.Token == "" {
+		return nil
+	}
+	if time.Now().Add(30 * time.Second).After(s.Expire) {
+		return nil
+	}
+	return &LoginAuth{Expire: s.Expire, Token: s.Token}
+}
+
+// Credentials returns login credentials, if possible, otherwise it
+// returns nil, meaning that you should create an account.
+func (s State) Credentials() *LoginCredentials {
+	if s.ClientID == "" {
+		return nil
+	}
+	if s.Password == "" {
+		return nil
+	}
+	return &LoginCredentials{ClientID: s.ClientID, Password: s.Password}
+}
+
+// StateFile is the orchestra state file. It is backed by
+// a generic key-value store configured by the user.
+type StateFile struct {
+	Store model.KeyValueStore
+	key   string
+}
+
+// NewStateFile creates a new state file backed by a key-value store
+func NewStateFile(kvstore model.KeyValueStore) StateFile {
+	return StateFile{key: "orchestra.state", Store: kvstore}
+}
+
+// SetMockable is a mockable version of Set
+func (sf StateFile) SetMockable(s State, mf func(interface{}) ([]byte, error)) error {
+	data, err := mf(s)
+	if err != nil {
+		return err
+	}
+	return sf.Store.Set(sf.key, data)
+}
+
+// Set saves the current state on the key-value store.
+func (sf StateFile) Set(s State) error {
+	return sf.SetMockable(s, json.Marshal)
+}
+
+// GetMockable is a mockable version of Get
+func (sf StateFile) GetMockable(sfget func(string) ([]byte, error),
+	unmarshal func([]byte, interface{}) error) (State, error) {
+	value, err := sfget(sf.key)
+	if err != nil {
+		return State{}, err
+	}
+	var state State
+	if err := unmarshal(value, &state); err != nil {
+		return State{}, err
+	}
+	return state, nil
+}
+
+// Get returns the current state. In case of any error with the
+// underlying key-value store, we return an empty state.
+func (sf StateFile) Get() (state State) {
+	state, _ = sf.GetMockable(sf.Store.Get, json.Unmarshal)
+	return
+}