[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/error.go

@@ -0,0 +1,39 @@
+package pluginapi
+
+import (
+	"net/http"
+
+	"github.com/pkg/errors"
+
+	"github.com/mattermost/mattermost/server/public/model"
+)
+
+// ErrNotFound is returned by the plugin API when an object is not found.
+var ErrNotFound = errors.New("not found")
+
+// normalizeAppErr returns a truly nil error if appErr is nil as well as normalizing a class
+// of non-nil AppErrors to simplify use within plugins.
+//
+// This doesn't happen automatically when a *model.AppError is cast to an error, since the
+// resulting error interface has a concrete type with a nil value. This leads to the seemingly
+// impossible:
+//
+//	var err error
+//	err = func() *model.AppError { return nil }()
+//	if err != nil {
+//	    panic("err != nil, which surprises most")
+//	}
+//
+// Fix this problem for all plugin authors by normalizing to special case the handling of a nil
+// *model.AppError. See https://golang.org/doc/faq#nil_error for more details.
+func normalizeAppErr(appErr *model.AppError) error {
+	if appErr == nil {
+		return nil
+	}
+
+	if appErr.StatusCode == http.StatusNotFound {
+		return ErrNotFound
+	}
+
+	return appErr
+}