[java-logging] Add support of AppHub with LogEntry.Builder

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**Is your feature request related to a problem? Please describe.**
I want to set the [apphub fields](https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#AppHub) when creating a log entry via the LogEntry.Builder. Only these fields doesn't seem to be supported via the Java client.

**Describe the solution you'd like**
Add support for the [Apphub fields](https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#AppHub) with the LogEntry.Builder.

**Describe alternatives you've considered**
Nope

**Additional context**
I'm creating a logging service which on behave of frontend client applications write log entries to Cloud Logging.