Refactor: streamline state handling in populatedb

[osmanhadzic]

Enhancements to state handling in tools/astarte_import/lib/astarte/import/populatedb.ex:

• Refactored the got_end_of_object_fun function to properly destructure and update the State structure.
• Ensured the last_seen_reception_timestamp is correctly updated within the got_end_of_property_fun function.

Improvements to structure handling:

• tools/astarte_import/lib/astarte/import/populatedb.ex: Modified the got_end_of_object_fun function to correctly destructure the State structure and update its fields.
• tools/astarte_import/lib/astarte/import/populatedb.ex: Ensured the last_seen_reception_timestamp is updated within the got_end_of_property_fun function by updating the data field of the Import.State structure.

• [x] I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md
• [ ] I have added to CHANGELOG.md relevant changes and any other user facing change
• [x] I have added or ran the appropriate tests

What this PR does / why we need it:

The main goal of these changes is to improve the integration and functionality of the astarte_import tool with the astarte_data_access module, ensuring efficient data access and processing within the Astarte platform.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

• [ ] Yes
• [x] No

Additional documentation e.g. usage docs, diagrams, etc.: