Hide members and make RecordSet picklable.

[panh99]

We previously expose both public and internal members of RecordSet. Users were being presented with pairs of properties for managing parameters, metrics, and configurations (parameters_records, _parameters_records; metrics_records, _metrics_records; configs_records, _configs_records). This redundancy not only cluttered the autocomplete suggestions but also appeared unprofessional and potentially confusing for users interacting with the API.

The PR solves this issue by hiding the internal members. In addition, it makes the RecordSet work with pickle.

Previous:

Improved:

[jafermarq]

I wonder if at least a super minimal test could be added to recordset_test.py:

def test_record_is_picklable():
    pickle.dumps(RecordSet())

This passes with the updates in this PR.