generate protocol-specific event payload

[AllanZhengYP]

Issue #, if available:
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 of changes:
This change expose a new interface serializeInputEventDocumentPayload() which generates the protocol-specific event payload. Specificly as mentioned in the Smithy spec:

A structure and union is serialized as a protocol-specific document.
It should only handle the serialization of structure and union shape into the event payload.

This change refactors the generator caused by the misunderstanding previously.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[AllanZhengYP]

Per discussing with @kstich, this PR doesn't handle the case where only a subset of members contains eventHeaders trait and the rest of the members contains implicit eventPayload trait. I need to rework on this PR.

[AllanZhengYP]

Address issue above in commit aws/aws-sdk-js-v3@ad2fa4e. The change excludes serializing the union or structure members with eventHeader trait. It means the protocol-specific serializer will only serialize the members in the event payload.

cc @kstich