[gRPC] Support well-known protobuf library types.

[witemple-msft]

Google maintains a core protobuf library that has some shared models that are required to support some data types well: https://developers.google.com/protocol-buffers/docs/reference/google.protobuf

For example:

• Map dateTime fields (only naive with respect to time zone) into google.protobuf.Timestamp.
• Map null to google.protobuf.NullValue.
• Map void to google.protobuf.Empty
• Maybe map unknown models to google.protobuf.Any

google.protobuf must be included somehow in the output directory (e.g. emitted by cadl-grpc alongside the user's packages) and types must be referenced using import declarations with a relative path, e.g.:

import "../../google/protobuf/empty.proto";

package "com.azure.Widgets";

service WidgetService {
  rpc paint (PaintRequest) returns (google.protobuf.Empty);
}

[witemple-msft]

gRPC priority: 1 (IMO, this is essential)
Estimate: 5
Needs Design - should we emit the required google protobuf library components as part of the CADL spec emit or require customers to provide the google protobuf libraries themselves. Should it be configurable?