[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/common/url.go

@@ -0,0 +1,28 @@
+package common
+
+import (
+	"net/url"
+	"strings"
+
+	"github.com/mattermost/mattermost/server/public/pluginapi"
+)
+
+// GetPluginURL returns a url like siteURL/plugins/pluginID based on the information from the client.
+// If any error happens in the process, a empty string is returned.
+func GetPluginURL(client *pluginapi.Client) string {
+	mattermostSiteURL := client.Configuration.GetConfig().ServiceSettings.SiteURL
+	if mattermostSiteURL == nil {
+		return ""
+	}
+	_, err := url.Parse(*mattermostSiteURL)
+	if err != nil {
+		return ""
+	}
+	manifest, err := client.System.GetManifest()
+	if err != nil {
+		return ""
+	}
+
+	pluginURLPath := "/plugins/" + manifest.Id
+	return strings.TrimRight(*mattermostSiteURL, "/") + pluginURLPath
+}