Google proto timestamps support

[guicassolato]

Adds support for selecting values that match google.golang.org/protobuf/types/known/timestamppb.Timestamp type (i.e. { "seconds": Number, "nanos": Number }) from the Authorization JSON. The source values are converted to their corresponding timestamp string representation in RFC3339 format.

Breaking change:

Users that previously trusted on selectors/CEL expressions such as request.time to resolve to { "seconds": Number, "nanos": Number } shall now expect RFC3339 instead.

Users can still dig into the details of a timestamp value in the middle of an expression or for extracting only one of the parts of the timestamp (e.g. request.time.seconds). However, RFC3339 will be used whenever extracting a value known to be a perfect representation of a timestamp in Google's protobuf format, for example, when building dynamic URLs and request parameters for fetching metadata from external sources, when extending properties of identity objects, and dynamic authorization response attributes (e.g. injected HTTP headers, etc) from these values.

Although google.golang.org/protobuf/types/known/timestamppb.Timestamp declares both seconds and nanos as optional. For the conversion to RFC3339 to trigger, at least one of these properties must be present in the source value, and no other property other than these two.