fix(util-dynamodb): add alternate type-correct APIs for marshalling

[kuhe]

Issue

#8154

Description

Creates a new codec API to replace the marshall/unmarshall API in util-dynamodb.
The old API is too entangled with broken type interfaces.

This avoids making a breaking-change fix in util-dynamodb.

However, lib-dynamodb changes to use the new codec API, and includes some breakfix changes, narrowing some types to only those that work with DynamoDB.

Testing

new unit tests and existing lib-dynamodb e2e tests.

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts) or E2E tests.
  • It's not a feature.
• My E2E tests are resilient to concurrent i/o.
  • I didn't write any E2E tests.
• I added access level annotations e.g. @public, @internal tags and enabled doc generation on the package. Remember that access level annotations go below the description, not above.
  • I didn't add any public functions.
• Streams - how do they work?? My WebStream readers/locks are properly lifecycled. Node.js stream backpressure is handled. Error handling.
  • No streams here.

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[kuhe]

• todo: codegen updates to lib-dynamodb

[kuhe]

• needs opt-in to new behavior