Submissions Search (by Kapp as POST)

Returns submissions for the specified Kapp. Searching requires kapp index definitions to exist that include the
properties and fields specified by the q and orderBy request body properties.

NOTE: This endpoint is functionally equivalent to GET /kapps/{kappSlug}/forms/{formSlug}/submissions, but can be
used if the search query string is expected to be significantly long (such as when searching with an IN
expression that has a large number of long values).