Improve Payload and apply to more data modality

[hunterhector]

Is your feature request related to a problem? Please describe.
We created Payload ontology when supporting vision data, yet this concept can be used for Audio and text too, which will allow we supporting multiple payloads per data pack.

Consider this an optional task for the 0.3 milestone

Describe the solution you'd like

1. Double check Payload implementation to make sure it is flexible for more modalities
2. Create one more layer of ontology supporting different modalities of payload (i.e. ImagePayload, TextPaylod), and make sure these ontology definitions define enough metadata (such as original media type, etc.)
3. Create utilities to load from disk and cache internally
4. Index payloads to make sure they can be accessed (named payload would be good too)

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Additional context
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