Add support of AppHub with LogEntry.Builder

[steven166]

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Is your feature request related to a problem? Please describe.
I want to set the apphub fields 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 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.