Implement fetching OpenID token for hosting provider iframe

And then pass it via postMessage when requested.

Send whole OpenID credentials object to Hosting Provider iframe

src/components/structures/HostingProviderDialog.tsx

@@ -16,17 +16,83 @@ limitations under the License.
 
 import * as React from "react";
 import SdkConfig from "../../SdkConfig";
+import {MatrixClientPeg} from "../../MatrixClientPeg";
 
 interface IProps {}
 
-interface IState {}
+interface IState {
+    error: string,
+}
 
 export default class HostingProviderDialog extends React.PureComponent<IProps, IState> {
+    iframeRef;
+    hostingSignupUrl: string;
+
+    constructor(props: IProps) {
+        super(props);
+
+        this.state = {
+            error: null,
+        };
+
+        this.iframeRef = React.createRef();
+        this.hostingSignupUrl = SdkConfig.get().hosting_signup_iframe;
+    }
+
+    private messageHandler = (message) => {
+        if (!this.hostingSignupUrl.startsWith(message.origin)) {
+            return;
+        }
+
+        switch (message.data.action) {
+            case 'openid_credentials_request':
+                // noinspection JSIgnoredPromiseFromCall
+                this.fetchOpenIDToken();
+                break;
+        }
+    }
+
+    private sendMessage = (message) => {
+        this.iframeRef.contentWindow.postMessage(
+            message,
+            this.hostingSignupUrl,
+        )
+    }
+
+    private async fetchOpenIDToken() {
+        const token = await MatrixClientPeg.get().getOpenIdToken();
+        if (token && token.access_token) {
+            this.sendMessage({
+                action: 'openid_credentials',
+                tokenData: token,
+            });
+        } else {
+            this.setState({
+                error: "Failed to connect to your homeserver. Please close this dialog and try again.",
+            });
+        }
+    }
+
+    public componentDidMount() {
+        window.addEventListener("message", this.messageHandler);
+    }
+
+    public componentWillUnmount() {
+        window.removeEventListener("message", this.messageHandler);
+    }
+
     public render(): React.ReactNode {
-        const hostingSignupUrl = SdkConfig.get().hosting_signup_iframe;
         return (
             <div className="mx_HostingProviderDialog_container">
-                <iframe src={hostingSignupUrl} />
+                <iframe
+                    src={this.hostingSignupUrl}
+                    ref={ref => this.iframeRef = ref}
+                />
+                {this.state.error &&
+                    <div>
+                        {this.state.error}
+                    </div>
+                }
             </div>
         );
     }