DynamoDB Single Table Design Operations.

Performs operations on DynamoDB tables following the single table design pattern.
Multiple entity types are stored in one table using composite primary keys (pk/sk)
and GSIs to support various access patterns.

Key conventions:
  - pk, pk1, pk2, ... : Partition key columns (pk for main table, pk1+ for GSIs)
  - sk, sk1, sk2, ... : Sort key columns (sk for main table, sk1+ for GSIs)
  - GSI naming: gsiPk{n}Sk{m} (e.g., gsiPkSk1, gsiPk1Sk2)
  - creationTime: Automatically set on first insert (immutable)
  - updateTime: Automatically updated on every upsert

Attribute values are marshalled to and from DynamoDB types automatically:
  - written: nil -> NULL, string -> S, number -> N, boolean -> BOOL,
    byte array / ByteBuffer -> B, map -> M, sequential -> L
  - read: the above plus SS and NS (as sets) and BS (as a vector of byte arrays)
Binary attributes are read back as byte arrays. Strings are always written as S,
never as binary. pk/sk values must not be binary.