fix(client-lex-runtime-v2): serialize eventstream payload properly

[AllanZhengYP]

Issue

Ref: P64550089
The Lex-runtime-v2 service cannot serialzie the event correctly because the events without eventHeader or eventPayload trait are not serialized, but returned as plain JavaScript object.

For example:

1. The TextEvent is modeled as structure and serialized into a JavaScript Object(link).
2. The JavaScript Object was directly passed to the event body(link) without serializing.
3. This results in the event marshaller fails to the allocate correct bytelength, because it assumes the body to be binary data(link)

Description

This change makes sure the event payload are serialized to Uint8Array before encoded by event stream marshaller.

Testing

Manually tested

Additional context

Associated to smithy-lang/smithy-typescript#554

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