[MM-53968] Includes mattermost-plugin-api into the mono repo (#24235)

Include https://github.com/mattermost/mattermost-plugin-api into the mono repo

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Michael Kochell <mjkochell@gmail.com>
Co-authored-by: Alejandro García Montoro <alejandro.garciamontoro@gmail.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Alex Dovenmuehle <alex.dovenmuehle@mattermost.com>
Co-authored-by: Michael Kochell <6913320+mickmister@users.noreply.github.com>
Co-authored-by: Christopher Poile <cpoile@gmail.com>
Co-authored-by: İlker Göktuğ Öztürk <ilkergoktugozturk@gmail.com>
Co-authored-by: Shota Gvinepadze <wineson@gmail.com>
Co-authored-by: Ali Farooq <ali.farooq0@pm.me>
Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>
Co-authored-by: Daniel Espino García <larkox@gmail.com>
Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Alex Dovenmuehle <adovenmuehle@gmail.com>
Co-authored-by: Szymon Gibała <szymongib@gmail.com>
Co-authored-by: Lev <1187448+levb@users.noreply.github.com>
Co-authored-by: Jason Frerich <jason.frerich@mattermost.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>
Co-authored-by: Artur M. Wolff <artur.m.wolff@gmail.com>
Co-authored-by: Madhav Hugar <16546715+madhavhugar@users.noreply.github.com>
Co-authored-by: Joe <security.joe@pm.me>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: José Peso <trilopin@users.noreply.github.com>

server/public/pluginapi/experimental/flow/state.go

@@ -0,0 +1,146 @@
+package flow
+
+import (
+	"bytes"
+	"errors"
+	"text/template"
+)
+
+var errStateNotFound = errors.New("flow state not found")
+
+// State is the "app"'s state
+type State map[string]interface{}
+
+func (s State) MergeWith(update State) State {
+	n := State{}
+	for k, v := range s {
+		n[k] = v
+	}
+	for k, v := range update {
+		n[k] = v
+	}
+	return n
+}
+
+// GetString return the value to a given key as a string.
+// If the key is not found or isn't a string, an empty string is returned.
+func (s State) GetString(key string) string {
+	vRaw, ok := s[key]
+	if ok {
+		v, ok := vRaw.(string)
+		if ok {
+			return v
+		}
+	}
+
+	return ""
+}
+
+// GetInt return the value to a given key as a int.
+// If the key is not found or isn't an int, zero is returned.
+func (s State) GetInt(key string) int {
+	vRaw, ok := s[key]
+	if ok {
+		v, ok := vRaw.(int)
+		if ok {
+			return v
+		}
+	}
+
+	return 0
+}
+
+// GetBool return the value to a given key as a bool.
+// If the key is not found or isn't a bool, false is returned.
+func (s State) GetBool(key string) bool {
+	vRaw, ok := s[key]
+	if ok {
+		v, ok := vRaw.(bool)
+		if ok {
+			return v
+		}
+	}
+
+	return false
+}
+
+// JSON-serializable flow state.
+type flowState struct {
+	// The name of the step.
+	StepName Name
+
+	Done bool
+
+	// ID of the post produced by the step.
+	PostID string
+
+	// Application-level state.
+	AppState State
+}
+
+func (f *Flow) storeState(state flowState) error {
+	if f.UserID == "" {
+		return errors.New("no user specified")
+	}
+
+	// Set AppState to differentiate an existing flow
+	if state.AppState == nil {
+		state.AppState = State{}
+	}
+
+	ok, err := f.api.KV.Set(kvKey(f.UserID, f.name), state)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return errors.New("value not set without errors")
+	}
+
+	f.state = &state
+	return nil
+}
+
+func (f *Flow) getState() (flowState, error) {
+	if f.UserID == "" {
+		return flowState{}, errors.New("no user specified")
+	}
+	if f.state != nil {
+		return *f.state, nil
+	}
+	state := flowState{}
+	err := f.api.KV.Get(kvKey(f.UserID, f.name), &state)
+	if err != nil {
+		return flowState{}, err
+	}
+	if state.AppState == nil {
+		return flowState{}, errStateNotFound
+	}
+
+	f.state = &state
+	return state, err
+}
+
+func (f *Flow) removeState() error {
+	if f.UserID == "" {
+		return errors.New("no user specified")
+	}
+	f.state = nil
+	return f.api.KV.Delete(kvKey(f.UserID, f.name))
+}
+
+func kvKey(userID string, flowName Name) string {
+	return "_flow-" + userID + "-" + string(flowName)
+}
+
+func formatState(source string, state State) string {
+	t, err := template.New("message").Parse(source)
+	if err != nil {
+		return source + " ###ERROR: " + err.Error()
+	}
+	buf := bytes.NewBuffer(nil)
+	err = t.Execute(buf, state)
+	if err != nil {
+		return source + " ###ERROR: " + err.Error()
+	}
+	return buf.String()
+}