Create a setting to select either PHP or the database for the summation [#3560573]

Problem/Motivation

This module was built using PHP to perform a sum on each row, but using the database is likely the more performant way to do this. Let's try adding a database option and giving users the choice of method.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork views_cumulative_field-3560573

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3560573-create-a-setting changes, plain diff MR !8
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