Add NativeChangesetReader API with filtering

[soham-bentley]

imodel-native: iTwin/imodel-native#1368

In Phase 1 (this PR):

• We give lazy property loading - using three property filters(InstanceKey, BisCoreElement, All)
• We can now open multiple changeset files in the form of a changeset group
• We can decode arrays properly.
• We use the standard EC-to-JSON adaptor
• We give data on a row by row basis using our JSON adaptor
• We support rich metadata like operation type and isIndirectChange flag
• We support filtering based on operation(Inserted or Updated or Deleted), table names and classNames.

[Copilot]

Pull request overview

Adds a new native-backed EC changeset/txn reader to core/backend and a unifier utility to merge per-table partial EC change rows into complete EC instances, with a standalone test validating openTxn() behavior against a locally-saved transaction.

Changes:

• Introduces ECChangesetReader (openFile/openGroup/openLocalChanges/openInMemoryChanges/openTxn) implementing ECNativeChangeSource.
• Adds ECNativePartialChangeUnifier plus in-memory and SQLite-backed cache implementations for merging partial instances.
• Adds a standalone test exercising openTxn() + unification for an inserted element and updated model rows.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
core/backend/src/ECChangesetReader.ts	New native changeset/txn reader + partial-change unifier + cache implementations.
core/backend/src/test/standalone/ECChangesetReader.test.ts	New standalone test validating openTxn() reading and unification results.

[soham-bentley]

@khanaffan @rschili I have added as much extensive testing as possible. But still if you fee something more can or should be tested
Please feel free to mention it in the PR

[soham-bentley]

I have taken absolutely separate code path in my new ChangesetReader and unifier API
Because the existing ChangesetECAdaptor and unifier API will be deprecated and removed in some time, so forcefully unifying the logic didn't make sense. It would have made the code unnecessarily complex. Instead this new unifier and ChangesetReader API and its exported types can become the standard going forward (if everybody agrees).