Expose presence of explicit optional keyword on fields

[mwatson2]

Whilst all fields in proto3 are optional, in the sense that they can be omitted from the message serialization, the optional keyword still carries a meaning. For fields that are not explicitly marked optional then when the field is missing from a message this is considered equivalent to the field being present with default value. APIs need not distinguish these two cases. When fields are explicitly marked optional (or if they are in a oneof) then the field missing case is a separate case from present-with-default-value case.

It would therefore be useful to expose whether a field is explicitly marked as optional in the parsed data structure.

This PR adds a property optional in the field structure that, if present and true, indicates that the field was marked explicitly as optional, or inside a oneof.