fix: message parameters in querystring

[bshaffer]

This PR addresses two issues happening in our REST query string serialization

1. Ensures when additionalBindings contains a message with nested fields, the fields are added to the querystring using dot syntax (e.g. interval.startTime)

   Example: in Monitoring V3, we require the interval query string parameter, but this is of type TimeInterval, and should actually end up being interval.startTime and interval.endTime (see the API documentation)

2. Ensures certain protobuf types are serialized to JSON correctly according to https://developers.google.com/protocol-buffers/docs/proto3#json

   Example: the values for startTime and endTime in the example above should be a formatted date string derived from the Google\Protobuf\Timestamp message.

Other things worth mentioning

• This will fix an error happening in the Monitoring library when a REST call is made to any RPC with a message object in the additionalBindings descriptors (e.g. listTimeSeries). The error being thrown is Object of class Google\Cloud\Monitoring\V3\TimeInterval could not be converted to string
• The nested bindings are calculated only one level deep. This means if a nested message has a message as a property, it will be a JSON-encoded string, and will probably not work as expected. This could be fixed with recursion, but I am being conservative on what we need to support here. Also, this new behavior is better than a PHP fatal error.