[Protobuf] Fix generation of "optional" labels in proto3 schema outW

[jeroenvandisseldorp]

When parsing a Protobuf schema, and then generating it back, fields marked with "optional" don't get the same label in the generated output. Instead, they become required. This patch checks per field if it needs to set the "proto3Optional" flag (ignored for oneOf fields). The flag is set if the field is optional, rendering the expected "optional" keyword in fileElement.toSchema().

[apicurio-bot]

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[jeroenvandisseldorp]

I'm working on Protobuf support for KSML, see https://github.com/Axual/ksml

The following code converts an incoming Protobuf schema to KSML's internal data structure, and then back into a Protobuf schema. The compatibility checker then verifies the schema is similar, ie. no differences. The supplied patch is required to force "optional" fields to receive the "optional" label in the generated output from FileElement.toSchema().

    private void runCompatibilityTest(String name, String schemaIn) {
        // This method checks for code completeness of KSML's ProtobufSchemaMapper. It will warn if schema
        // translation to KSML's internal DataSchema and back gives deltas between input and output ProtobufSchemas.
        final var proto = new ProtobufSchemaParser<>().parseSchema(schemaIn.getBytes(), Collections.emptyMap());
        final var internalSchema = MAPPER.toDataSchema(name, proto);
        final var out = MAPPER.fromDataSchema(internalSchema);
        final var outFd = out.getFileDescriptor();
        final var outFe = new ProtobufSchemaParser<>().toProtoFileElement(outFd);
        final var schemaOut = outFe.toSchema();
        final var checker = new ProtobufCompatibilityCheckerLibrary(new ProtobufFile(schemaIn), new ProtobufFile(schemaOut));
        final var diffs = checker.findDifferences();
        final var compatible = diffs.isEmpty();
        if (!compatible) {
            log.warn("PROTOBUF schema {} in/out is not compatible: {}", name, diffs);
        }
    }