[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>
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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>
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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/command/command.go

@@ -0,0 +1,31 @@
+package command
+
+import (
+	"encoding/base64"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+)
+
+// PluginAPI is the plugin API interface required to manage slash commands.
+type PluginAPI interface {
+	GetBundlePath() (string, error)
+}
+
+// GetIconData returns the base64 encoding of a icon for a given path.
+// The data returned may be used for slash command autocomplete.
+func GetIconData(api PluginAPI, iconPath string) (string, error) {
+	bundlePath, err := api.GetBundlePath()
+	if err != nil {
+		return "", errors.Wrap(err, "couldn't get bundle path")
+	}
+
+	icon, err := os.ReadFile(filepath.Join(bundlePath, iconPath))
+	if err != nil {
+		return "", errors.Wrap(err, "failed to open icon")
+	}
+
+	return fmt.Sprintf("data:image/svg+xml;base64,%s", base64.StdEncoding.EncodeToString(icon)), nil
+}