feat(core): Make \matcher\ parameter optional in \makeMultiplexedTransport\

The `matcher` parameter in `makeMultiplexedTransport` is now optional with a
sensible default. This makes it much easier to use the multiplexed transport for
sending events to multiple DSNs based on runtime configuration.

**Before:**

```javascript
import { makeFetchTransport, makeMultiplexedTransport } from '@sentry/browser';

const EXTRA_KEY = 'ROUTE_TO';

const transport = makeMultiplexedTransport(makeFetchTransport, args => {
  const event = args.getEvent();
  if (event?.extra?.[EXTRA_KEY] && Array.isArray(event.extra[EXTRA_KEY])) {
    return event.extra[EXTRA_KEY];
  }
  return [];
});

Sentry.init({
  transport,
  // ... other options
});

// Capture events with routing info
Sentry.captureException(error, {
  extra: {
    [EXTRA_KEY]: [
      { dsn: 'https://[email protected]/project1', release: 'v1.0.0' },
      { dsn: 'https://[email protected]/project2' },
    ],
  },
});
```

**After:**

```javascript
import {
  makeFetchTransport,
  makeMultiplexedTransport,
  MULTIPLEXED_TRANSPORT_EXTRA_KEY
} from '@sentry/browser';

// Just pass the transport generator - the default matcher handles the rest!
Sentry.init({
  transport: makeMultiplexedTransport(makeFetchTransport),
  // ... other options
});

// Capture events with routing info using the exported constant
Sentry.captureException(error, {
  extra: {
    [MULTIPLEXED_TRANSPORT_EXTRA_KEY]: [
      { dsn: 'https://[email protected]/project1', release: 'v1.0.0' },
      { dsn: 'https://[email protected]/project2' },
    ],
  },
});
```

The default matcher looks for routing information in
`event.extra[MULTIPLEXED_TRANSPORT_EXTRA_KEY]`. You can still provide a custom
matcher function for advanced use cases.

Author: maxkosty

packages/core/src/transports/multiplexed.ts

@@ -151,3 +151,27 @@ export function makeMultiplexedTransport<TO extends BaseTransportOptions>(
     };
   };
 }
+
+export const MULTIPLEXED_TRANSPORT_EXTRA_KEY = "multiplexed_transport";
+
+/**
+ * Creates a transport that will send events to all DSNs provided in event.extra["multiplexed_transport"]
+ * in the format of [{dsn: "__MY_DSN__", release: "__MY_RELEASE__"}, ...]. If no such key exists or list 
+ * is empty event will be sent to main DSN provided in Sentry.init().
+ */
+export function makeSimpleMultiplexedTransport<TO extends BaseTransportOptions>(
+  createTransport: (options: TO) => Transport,
+): (options: TO) => Transport {
+  return makeMultiplexedTransport(makeFetchTransport, (args) => {
+    const event = args.getEvent();
+    if (
+      event &&
+      event.extra &&
+      MULTIPLEXED_TRANSPORT_EXTRA_KEY in event.extra &&
+      Array.isArray(event.extra[MULTIPLEXED_TRANSPORT_EXTRA_KEY])
+    ) {
+      return event.extra[MULTIPLEXED_TRANSPORT_EXTRA_KEY];
+    }
+    return [];
+  });
+}