Store durations with option keeps the timing of already run testcases

## Description
Thanks for the great lib, but I would like to have the option to keep the timing of the tests that have already been run. This would allow me to avoid re-update tests that I know will not change much, and it would also make it easier to track the performance of my test suite over time in git.

## Solution Proposed
I think this could be implemented by update option `store_durations` into category: [keep, replace], and it would be default to `replace`.

I believe this would be a useful feature for many users of pytest-split and if you think the proposed solution is good to go. I happy to contribute and create a PR for this feature.

Start by locating the update option and its store_durations handling in pytest-split, then inspect the existing tests for duration updates. Implement keep and replace behavior, with replace remaining the default, and verify that already-run testcase timings are preserved when keep is selected.

Written by the indexing model from the issue text.