[API/feature?] make fids caching optional

[roya0045]

Description

This is a follow up PR to #31061 that enabled caching of fids per symbol during the counting procedure.

This PR aims to make fids caching optional, as it is not necessary to have the have the fids in most cases, especially until #30297 is merged.

This should prevent for useless memory usage.

If this is merged after 3.10 is published, backporting is highly recommended to prevent extra memory usage.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[roya0045]

@nyalldawson @m-kuhn I'm not sure if this is a feature or a preventive bugfix but this would be nice to have it in 3.10 in case some users have to do counting on big dataset.

[roya0045]

@m-kuhn a quick review would be appreciated.

[roya0045]

Backport would be preferable, especially early in this cycle.

[m-kuhn]

Looks good to me, just some minor changes to wording.
And +1 for backporting.

[roya0045]

@m-kuhn Thanks for the review/suggestions. I have also made the same changes to the counter in order to have constant docs.

[m-kuhn]

Thanks a lot

[roya0045]

@m-kuhn Did I need to do anything for the backporting? I assume a manual backport is only needed when automatic backporting fails.