feat(lib-transfer-manager): add download directory functionality

[smilkuri]

Issue

Internal JS-7054

Description

Implements the S3 Transfer Manager's downloadDirectory API. It downloads all objects from a given S3 bucket under a given prefix to a local directory, with optional filtering and per-request customization.

Testing

Added unit tests and 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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