[Feature Request] Support index sorting for indices with nested fields

[jhinch-at-atlassian-com]

Is your feature request related to a problem? Please describe

Currently index sorting and nested fields are mutually exclusive features (validated by DocumentMapper). However it should be possible to allow them to be used together under certain constraints.

Describe the solution you'd like

Indexing sorting should be possible if the fields used for sorting are not on the nested document and are instead only on the parent document. This would allow for nested documents to have subsequent doc IDs immediately after the parent document

Related component

Indexing

Describe alternatives you've considered

No response

Additional context

Its not clear to me if simply tweaking the validation logic would be sufficient or if there are additional changes required when applying the index sort.