feat: signals plugins

next try after #18153 without any external dependencies and only
supporting websocket. missing tests and necessary documentation but will
work on those after initial comments if this would be proper way to go
forward. already planning for the notification plugins on top of this.

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/signals-node/package.json

@@ -0,0 +1,42 @@
+{
+  "name": "@backstage/plugin-signals-node",
+  "description": "Node.js library for the signals plugin",
+  "version": "0.0.0",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "license": "Apache-2.0",
+  "publishConfig": {
+    "access": "public",
+    "main": "dist/index.cjs.js",
+    "types": "dist/index.d.ts"
+  },
+  "backstage": {
+    "role": "node-library"
+  },
+  "scripts": {
+    "build": "backstage-cli package build",
+    "lint": "backstage-cli package lint",
+    "test": "backstage-cli package test",
+    "clean": "backstage-cli package clean",
+    "prepack": "backstage-cli package prepack",
+    "postpack": "backstage-cli package postpack"
+  },
+  "devDependencies": {
+    "@backstage/cli": "workspace:^",
+    "@types/express": "^4.17.21"
+  },
+  "files": [
+    "dist"
+  ],
+  "dependencies": {
+    "@backstage/backend-common": "workspace:^",
+    "@backstage/config": "workspace:^",
+    "@backstage/plugin-auth-node": "workspace:^",
+    "@backstage/plugin-events-node": "workspace:^",
+    "@backstage/types": "workspace:^",
+    "express": "^4.17.1",
+    "uuid": "^8.0.0",
+    "winston": "^3.2.1",
+    "ws": "^8.14.2"
+  }
+}