Add support for multi-attribute keys in DDB Mapper

[ianbotsf]

Issue #

Related to #472

Description of changes

This change adds support for multi-attribute keys in DynamoDB Mapper. Specifically, the KeySpec class is now generic over KeyType, which represents key values consisting of 1-4 attributes. Corresponding changes are made in various schema modeling classes to compensate. Additionally, KeyFilter has been updated to accept a KeyType for partition keys and distinct SortKeyExpr values for up to 4 sort key attributes.

When reviewing these changes, I recommend the following order:

• Mapper runtime:
  • KeyType and associated implementations/extensions — ADT for key values of 1-4 attributes
  • KeySpec and associated implementations/extensions — updated to be generic over KeyType instead of an unbound type K
  • ItemSchema — updated KeySpec usage
  • The model package — updated key usage in various modeling types (e.g., Table, PersistenceSpec, etc.)
  • KeyFilter and its implementation — updated to use KeyType for multi-attribute key queries
• Schema codegen:
  • MapperTypes — various refactors to reference new/updated runtime declarations
  • SchemaRenderer — various updates to generate updated schemas
• (...and then everything else...)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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• No codegen difference in the AWS SDK