Add optional image hosting for non-public Navidrome instances

When Navidrome is behind a private network (e.g. Tailscale), Discord
cannot fetch artwork URLs. This adds optional integration with imgbb
(24h expiry, requires API key) and uguu.se (3h expiry, no key needed)
to upload cover art to a public host.

Closes #1

# Conflicts:
#	go.mod
#	go.sum

plugin_suite_test.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"testing"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+func TestDiscordPlugin(t *testing.T) {
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "Discord Plugin Main Suite")
+}