protobuf: Enable all conformance tests

[Jozott00]

Subsystem
Protobuf

Problem Description
Proto2 conformance tests didn't compile and only proto3 and proto3 editions tests were activated.

Solution
This PR fixes all compilation errors and activate all conformance tests.
Additionally the ConformanceTest test runner now excludes all JSON related tests, as JSON is currently not supported.

The current conformance results are (JSON tests excluded):

=== Conformance Test Results (filtered) ===
Total baseline tests (filtered):   2805
[+] Passed tests:                  2790
[-] Failed tests:                  15

The 15 failing tests are:

1. Missing unknown fields in response message (we currently always skip unknown fields)

FAILED: 'Required.Proto2.ProtobufInput.UnknownOrdering.ProtobufOutput' with: Unknown field mismatch
FAILED: 'Required.Proto2.ProtobufInput.UnknownVarint.ProtobufOutput' with: Output was not equivalent to reference message: Expect: \250\037\001, but got:
FAILED: 'Required.Proto3.ProtobufInput.UnknownOrdering.ProtobufOutput' with: Unknown field mismatch
FAILED: 'Required.Proto3.ProtobufInput.UnknownVarint.ProtobufOutput' with: Output was not equivalent to reference message: Expect: \250\037\001, but got:
FAILED: 'Required.Editions_Proto2.ProtobufInput.UnknownOrdering.ProtobufOutput' with: Unknown field mismatch
FAILED: 'Required.Editions_Proto2.ProtobufInput.UnknownVarint.ProtobufOutput' with: Output was not equivalent to reference message: Expect: \250\037\001, but got:
FAILED: 'Required.Editions_Proto3.ProtobufInput.UnknownOrdering.ProtobufOutput' with: Unknown field mismatch
FAILED: 'Required.Editions_Proto3.ProtobufInput.UnknownVarint.ProtobufOutput' with: Output was not equivalent to reference message: Expect: \250\037\001, but got:

2. Missing support for extensions and option message_set_wire_format

FAILED: 'Required.Editions.ProtobufInput.ValidDelimitedExtension.GroupLike.ProtobufOutput' with: Output was not equivalent to reference message: deleted: (protobuf_test_messages.editions.groupliketype): { c: 99 }
FAILED: 'Required.Editions.ProtobufInput.ValidDelimitedExtension.NotGroupLike.ProtobufOutput' with: Output was not equivalent to reference message: deleted: (protobuf_test_messages.editions.delimited_ext): { c: 99 }
FAILED: 'Required.Proto2.ProtobufInput.MessageSetEncoding.UnknownExtension.ProtobufOutput' with: Output was not equivalent to reference message: deleted: message_set_correct.4135300[0]: "\000c"
FAILED: 'Recommended.Proto2.ProtobufInput.ValidMessageSetEncoding.SubmessageEncoding.NotUnknown.ProtobufOutput' with: Output was not equivalent to reference message: deleted: message_set_correct.(protobuf_test_messages.proto2.TestAllTypesProto2.E
FAILED: 'Recommended.Proto2.ProtobufInput.ValidMessageSetEncoding.SubmessageEncoding.ProtobufOutput' with: Output was not equivalent to reference message: deleted: message_set_correct.(protobuf_test_messages.proto2.TestAllTypesProto2.M
FAILED: 'Required.Proto2.ProtobufInput.ValidMessageSetEncoding.OutOfOrderGroupsEntries.ProtobufOutput' with: Output was not equivalent to reference message: deleted: message_set_correct.(protobuf_test_messages.proto2.TestAllTypesProto2.M
FAILED: 'Required.Proto2.ProtobufInput.ValidMessageSetEncoding.ProtobufOutput' with: Output was not equivalent to reference message: deleted: message_set_correct.(protobuf_test_messages.proto2.TestAllTypesProto2.M

[Mr3zee]

Hell yeah!