[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/panel/settings/setting.go

@@ -0,0 +1,33 @@
+package settings
+
+import (
+	"github.com/mattermost/mattermost/server/public/model"
+)
+
+const (
+	// ContextIDKey defines the key used in the context to store the ID
+	ContextIDKey = "setting_id"
+	// ContextButtonValueKey defines the key used in the context to store a button value
+	ContextButtonValueKey = "button_value"
+	// ContextOptionValueKey defines the key used in the context to store a selected option value
+	ContextOptionValueKey = "selected_option"
+
+	// DisabledString defines the string used to show that a setting is disabled
+	DisabledString = "Disabled"
+	// TrueString codify the boolean true into a string
+	TrueString = "true"
+	// FalseString codify the boolean false into a string
+	FalseString = "false"
+)
+
+// Setting defines the behavior of each element a the panel
+type Setting interface {
+	Set(userID string, value interface{}) error
+	Get(userID string) (interface{}, error)
+	GetID() string
+	GetDependency() string
+	IsDisabled(foreignValue interface{}) bool
+	GetTitle() string
+	GetDescription() string
+	GetSlackAttachments(userID, settingHandler string, disabled bool) (*model.SlackAttachment, error)
+}