chore(types): add types for EventStreamMarshaller ser/deser functions

[trivikr]

Issue

Internal JS-3350

Description

Adds types for EventStreamMarshaller ser/deser functions

Testing

Integration test on client-transcribe-streaming is successful:

$ client-transcribe-streaming> yarn test:integration
yarn run v1.22.19
$ jest --config jest.integ.config.js
 PASS  test/index.integ.spec.ts (32.259 s)
  TranscribeStream client
    ✓ should stream the transcript (28982 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        32.317 s
Ran all test suites.
Done in 33.61s.

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