Make proto3 optional fields into option types

[andersfugmann]

This change changes proto3 optional fields to be option types instead of an polymorphic variant. Currently proto3 optional fields are mapped as a oneof _<field_name> { <field_type> <field_name> = 1}. This maps to a polymorphic variant:

type t = { _<field_name>: [ `not_set | `<field_name> of <field_type> ]; ... }

The change proposed is to make optional fields in the same way as proto2:

type t = { <field_name>: <field_type> option; ... } 

[andersfugmann]

Apparently the github ubuntu runners does not have a sufficiently recent version of protoc (>= 3.12 required). Ill look into that later.

[andersfugmann]

Even worse - The tests would not even complete on any system with protoc < 3.12. Maybe we should just remove the extra tests (or somehow leave them as optional).

[andersfugmann]

Ok. Here is a fix for this little test problem and older compilers. By detecting if protoc compiler supports the new flag (I assume the flag will still be supported for protoc >= 3.15, where the flag is enabled by default), the build either includes or excludes testing the proto3_optional test.

Its convoluted, but it works, and its the simplest way I could think of. One drawback though is that the expect test is not really what we want, but the test will fail if proto3 options are not handled correctly.

[andersfugmann]

Thanks for reviewing. I will merge once the tests are green.

[andersfugmann]

I can create a new release over the weekend. I see you already tagged a v4.3. Should I override this tag, or create a new release tag?

[AndreasDahl]

Feel free to overwrite the 4.3.0 tag. I was intending to release it as-is, but I think we should rather have your changes in first. This was my release PR https://github.com/ocaml/opam-repository/pull/22085

[andersfugmann]

https://github.com/ocaml/opam-repository/pull/22090