feat(user-agent): add User-Agent plugin (#1582)

* feat(user-agent): add User-Agent plugin

* feat(user-agent): Few changes upon request

src/@ionic-native/plugins/user-agent/index.ts

@@ -0,0 +1,73 @@
+import { Injectable } from '@angular/core';
+import { Plugin, Cordova, IonicNativePlugin } from '@ionic-native/core';
+
+/**
+ * @name User Agent
+ * @description
+ * The UserAgent plugin provides functions to set the HTTP  user-agent header. For more info about User-Agents, please [see the HTTP User-Agent docs](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent).
+ *
+ * Requires Cordova plugin: `cordova-useragent`. For more info, please see the [User-Agent plugin docs](https://github.com/LouisT/cordova-useragent).
+ *
+ * @usage
+ * ```typescript
+ * import { UserAgent } from '@ionic-native/user-agent';
+ *
+ *
+ * constructor(private userAgent: UserAgent) { }
+ *
+ * ...
+ *
+ *
+ * this.userAgent.set('Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36')
+ *   .then((res: any) => console.log(res))
+ *   .catch((error: any) => console.error(error));
+ *
+ * * this.userAgent.get()
+ *   .then((res: any) => console.log(res))
+ *   .catch((error: any) => console.error(error));
+ *
+ * * this.userAgent.reset()
+ *   .then((res: any) => console.log(res))
+ *   .catch((error: any) => console.error(error));
+ *
+ * ```
+ */
+@Plugin({
+  pluginName: 'UserAgent',
+  plugin: 'https://github.com/LouisT/cordova-useragent',
+  pluginRef: 'plugins.useragent',
+  repo: 'https://github.com/LouisT/cordova-useragent',
+  platforms: ['Android', 'iOS']
+})
+@Injectable()
+export class UserAgent extends IonicNativePlugin {
+
+  /**
+   * Changes the current user-agent to the one sent by argument.
+   * @param userAgent {string} User-Agent
+   * @return {Promise<any>} Returns a promise that resolves when the user-agent changes
+   */
+  @Cordova()
+  set(userAgent: string): Promise<any> {
+    return;
+  }
+
+  /**
+   * Returns the current user-agent string.
+   * @return {Promise<any>} Returns a promise that resolves when the user-agent is returned
+   */
+  @Cordova()
+  get(): Promise<any> {
+    return;
+  }
+
+  /**
+   * Sets the user-agent back to default
+   * @return {Promise<any>} Returns a promise that resolves when the user-agent resets
+   */
+  @Cordova()
+  reset(): Promise<any> {
+    return;
+  }
+
+}