feat: use signals to update user settings over sessions

closes #24981

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

plugins/user-settings-common/.eslintrc.js

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+module.exports = require('@backstage/cli/config/eslint-factory')(__dirname);

plugins/user-settings-common/README.md

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+# @backstage/plugin-user-settings-common
+
+Welcome to the common package for the user-settings plugin!
+
+_This plugin was created through the Backstage CLI_

plugins/user-settings-common/api-report.md

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+## API Report File for "@backstage/plugin-user-settings-common"
+
+> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
+
+```ts
+// @public (undocumented)
+export type UserSettingsSignal = {
+  type: 'key-changed' | 'key-deleted';
+  key: string;
+};
+
+// (No @packageDocumentation comment for this package)
+```

plugins/user-settings-common/catalog-info.yaml

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+apiVersion: backstage.io/v1alpha1
+kind: Component
+metadata:
+  name: backstage-plugin-user-settings-common
+  title: '@backstage/plugin-user-settings-common'
+  description: The Backstage common plugin to manage user settings
+spec:
+  lifecycle: experimental
+  type: backstage-common-library
+  owner: maintainers

plugins/user-settings-common/package.json

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+{
+  "name": "@backstage/plugin-user-settings-common",
+  "version": "0.0.0",
+  "description": "Common functionalities for the user-settings plugin",
+  "backstage": {
+    "role": "common-library"
+  },
+  "publishConfig": {
+    "access": "public",
+    "main": "dist/index.cjs.js",
+    "module": "dist/index.esm.js",
+    "types": "dist/index.d.ts"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/backstage/backstage",
+    "directory": "plugins/user-settings-common"
+  },
+  "license": "Apache-2.0",
+  "sideEffects": false,
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "backstage-cli package build",
+    "clean": "backstage-cli package clean",
+    "lint": "backstage-cli package lint",
+    "prepack": "backstage-cli package prepack",
+    "postpack": "backstage-cli package postpack",
+    "test": "backstage-cli package test"
+  },
+  "devDependencies": {
+    "@backstage/cli": "workspace:^"
+  }
+}

plugins/user-settings-common/src/index.ts

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+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export * from './types';

plugins/user-settings-common/src/setupTests.ts

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+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+export {};

plugins/user-settings-common/src/types.ts

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+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/** @public */
+export type UserSettingsSignal = {
+  type: 'key-changed' | 'key-deleted';
+  key: string;
+};