Fix hard-coded model export dir name

[hongye-sun]

The change includes:

• Expose optional model payload in the interface
• Fix exporter folder searching logic to not depend on hard-coded export dir name.

The new searching logic assumes that user will provide the model_uri is either the direct parent folder of a model file or the base export dir, which contains a list of timestamped sub-directories.

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This change is 

[Ark-kun]

direct parent folder of a model file

Is a model a single file? Can model_uri also point to that file then?

/lgtm

[hongye-sun]

Direct link to model file doesn't work today. Only parent directory is supported.

[hongye-sun]

/approve

[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hongye-sun

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• component_sdk/OWNERS [hongye-sun]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment