Upgrading to Pydantic v2 and DiffSync v2

[theandrelima]

Overview

network-importer's dependency on pydantic = "^1.6" limits users who need to use other packages dependent on Pydantic 2.x in the same Python environment.

The original motivation behind this PR was to move network-importer's dependency to Pydantic v2.x. However, network-importer depends on DiffSync, which in turn also depends on Pydantic.

For that, moving to DiffSync v2 is also required for compatibility with Pydantic v2.

Changes Made

• Upgraded to Pydantic v2.
• Refactored settings classes to use pydantic-settings.
• Applied overall code fixes for Pydantic v2 compatibility.
• Upgraded to DiffSync v2 and made necessary code adjustments for breaking changes.
• Proposed project version bump to 4.0.0 (subject to NTC's discretion) due to breaking changes.

Testing

• All tests passed using the invoke pytest command. (see Additional Notes below)
• Manual testing in CLI mode against a dev instance of our Nautobot environment confirmed the functionality both in check and apply modes, and with/without --update-configs and --limit flags.

Additional Notes

invoke pytest only worked after I added the following line to the end of the Dockerfile: RUN pip install .

I'm not sure why, but lines 14 and 15 of the current Dockerfile were not taking the effect of making network_importer itself available in the python environment of the resulting built image.

I decided to not tackle this at all within this PR because:
1 - It might be some weirdness with my own sandbox environment.
2 - Even if it is not, I believe it is out-of-scope.