[API/feature?] Cache feature ids for each symbol in the QgsVectorLayer

[roya0045]

Description

This is a proposal to cache the feature ids in the vector layer, similar to what is currently done with the counts.

The method to the the Fids is an extra in case someone would need it eventually.

This replaces the PR #30950 where it was proposed to use the counter directly but was discarded due to thread safety issues as highlighted by @nyalldawson

The need to have the fids comes from the symbology expression PRs where I need to provide the fids of the features to the expression and I need to store and access this list of fids from somewhere when the aggregate function is evaluated.

Any feedback is appreciated

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and contain sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[roya0045]

@nyalldawson Your opinion would be appreciated on the subject.

Is this caching fine with you and does the function clutter the QgsVectorLayer APi too much or is this tolerable?

[roya0045]

@m-kuhn are you ok with this or do you still want to see what others have to say?

[roya0045]

@m-kuhn @nyalldawson any opinion on this proposal?

[roya0045]

@3nids PR has been rebased and squashed