Implement validation for incoming events

[rdner]

We should not pass an event if it has no Source, Timestamp or DataStream fields.

[rdner]

After a meeting, @cmacknz @faec and I decided on the following steps:

• We add a special Strict Mode configuration parameter for the gRPC server configuration
• If the Strict Mode is active, we inspect the whole event batch before trying to publish any of the events to the queue. We validate against missing required fields in each event.
• When we find an invalid event, we immediately return a codes.InvalidArgument gRPC error with a detailed error message that describes invalid fields of the event letting know the client as a failure
• When we're not in the Strict Mode there is no validation whatsoever, all the invalid events are accepted and propagated through the queue

The intention here is to have a "fail fast" approach for the input developers while debugging and to avoid additional validation overhead in production.