Bad publishTime format used when publishTime not provided in the message context

Current format:
```php
gmdate('%Y-%m-%dT%H:%M:%S.%6NZ')
```
That produces, for example, this string `%2023-%09-%29GMT%13:%Sep:%th.%650` instead of the [RFC 3339](https://tools.ietf.org/html/rfc3339) format.

It's an easy fix; I'll send a Pull Request. This might also be relevant for https://github.com/GoogleCloudPlatform/functions-framework-php/issues/66

---

I came across this when testing my cloud function with the PubSub emulator. There was a similar issue in the node implementation: https://github.com/googleapis/google-cloud-node/issues/1925.

Ultimately, it would be great if the emulator included this property. Is there a right place to file an issue with the PubSub emulator?
I assume that this is beyond the scope of this PHP repo, nonetheless, from my tests, it seems like there are situations where the `publishTime` is present in the PubSub message (for example when I'm pulling the message via the PubSub API), but not present when the payload reaches the cloud functions framework.