fix(ts): make template types consistent

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Summary

Follow up of #4774

In some cases, we were exporting our Templates types as required for our widgets, while they are in reality optional and partial.

To avoid our users to provide extra logic over our types and to keep it consistent with our usage, the types are now:

• Exported as optional in the widget.
• Export as required in the component.

Result

The exported types are now correct.

Ticket

DX-1778

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[Haroenv]

overall almost good to merge, although the Partial<{...}> should probably be stacked + deciding what to do with spreading defaultTemplates or equaling to defaultTemplates

[Haroenv]

all right, this is a great effort! let's still do a global search for userTemplates and avoid it wherever the templates get prepared after :)

[Haroenv]

perfect!