Reflect on Presence for proto3

According to the discussion in https://github.com/google/protobuf.dart/issues/490, this lib does generally support reflection using `generatedMessage.info_`.
However, I could not find reflection on explicit presence of fields.
The [protobuf docs](https://github.com/protocolbuffers/protobuf/blob/main/docs/implementing_proto3_presence.md#updating-reflection) recommends how to update reflection support regarding proto3 presence.

I am especially interested in support for `FieldInfo.has_presence`.

Start with the discussion in issue #490 and the protobuf proto3 presence reflection guidance linked here. Inspect how reflection is exposed through generatedMessage.info_ and determine where FieldInfo.has_presence should be represented. Done means explicit proto3 presence is available through the library's reflection support.

Written by the indexing model from the issue text.