chore: introduce memorystorev2

[cwaldren-ld]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Describe the solution you've provided

This adds a memorystorev2 package containing an alternative Memory Store implementation designed specifically for FDv2 usage.

It is similar to the existing memory store, with obsolete methods removed and new methods added - but the core upsert/init logic remains the same.

I've converted the existing unit tests and benchmarks to the new store.

Describe alternatives you've considered

Augment the existing memory store with FDv2 functionality. I did this at first, and although it worked, it wasn't clean. It meant the store needed to retain obsolete functions, and it wasn't clear how the FDv2 methods interacted with FDv1 methods. Example: SetBasis vs Init, seems redundant.

Additional context

This component is necessary for the dual-mode persistent/memory store PR.