DataPack setting for Modality data should support setting data in a new payload

[hepengfe]

Is your feature request related to a problem? Please describe. Current datapack setting for modality data only can add new payload for the first one. We expect setter can add payload to a new index.

For example, suppose we have 2 text payloads with text payload indices 0, 1. We should support datapack.set_text("text", 2).

Describe the solution you'd like

• we should support datapack.set_text("text", -1) which is equivalent to append text payload
• we should also support datapack.set_text("text") which the text payload index is by default -1

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.