fix(core): close AdvancedExtension serde gaps

[nielspardon]

I noticed there are multiple gaps in the serialization and deserialization of `AdvancedExtension which I'm trying to close with this PR:

• in PlanProtoConverter pass ExtensionProtoConverter to RelProtoConverter
• in ProtoPlanConverter pass ProtoExtensionConverter to ProtoRelConverter
• move/add HasExtension interface to AbstractDdlRel, AbstractWriteRel
• add missing serde logic for relations to RelProtoConverter, ProtoRelConverter
• adds a test that confirms the extension converters are passed through from plan converters to rel converters

[nielspardon]

I'm working on adding more tests to verify the serialization/deserialization for each relation type