Add class method Timestamp.from_time to ruby well known types

[mcclymont]

I was surprised to find out that from_time was not a class method, but an instance method.
This limits its usefulness, as to create a new Timestamp you have to first instantiate the object, then call the from_time method, and then you need to return/use the saved instance, because the from_time method returns the self.nanos value, rather than the timestamp itself.

Code example

Before

{
  updated_at: Google::Protobuf::Timestamp.new.tap { |t| t.from_time(updated_at) },
}

After

{
  updated_at: Google::Protobuf::Timestamp.from_time(updated_at),
}

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[TeBoring]

@mcclymont, could you sign the cla?

[mcclymont]

@googlebot I signed it!

[mcclymont]

@TeBoring What's the next step? I don't understand the CI failures

[deannagarcia]

Sorry for the delay, I'm rerunning tests now and will let you know if there's anything that needs fixed.

[deannagarcia]

Thanks for the addition, merging now!