Add index based nested sort_mode, that allows the sort by fields from specified nested inner objects identified by an index. This can be a useful sort_mode if the order of the nested inner objects has a meaning.

The index based sort_mode should look like this:

• index_1 - Instructs the nested sorting to only look at the second element.
• index_9 - Instructs the nested sorting to only look at the tenth element.
• index_first - Instructs the nested sorting to only use the first nested inner object.
• index_last - Instructs the nested sorting to only use the last nested inner object.

Relates to #2662