feature/dynamodb/attributevalue needs a new major version to remove all of the opt-in flags that should be default

[lucix-aws]

We've had to add all of the following enhancements/fixes through opt-in flags on encode/decode options to avoid potentially breaking customer data.

All of these behaviors should realistically be on by default, which we should do in a 2.x major version of the feature/dynamodb/attributevalue module.

We CANNOT do this right now due to the internal mirror of the Go v2 SDK being a monolithic module. That would force the behavioral break on every internal customer. We would need to address that before bumping the major version here.

• dynamodb: StringSet and StringSet decoders call unmarshaller incorrectly #2895
• Use encoding.TextMarshaler and encoding.TextUnmarshaler in DynamoDB attributevalue marshalers/unmarshalers, if available #2596
• Custom Marshaler returning NULL should be omitted from .MarshalMap() with omitempty tag #2731