reintroduce str as input to group_by in aggregate

[tsmith023]

This PR reintroduces using just string syntax when performing a aggregate group by query so that the happy path of usage is easier as it was previously, e.g.:

res = collection.aggregate.over_all(
    group_by="text",
    ...
)

instead of:

res = collection.aggregate.over_all(
    group_by=weaviate.classes.aggregate.GroupByAggregate(prop="text"),
    ...
)

[codecov-commenter]

Codecov Report

Attention: 277 lines in your changes are missing coverage. Please review.

Comparison is base (e977081) 93.48% compared to head (657ce0a) 95.50%.
Report is 41 commits behind head on main.

Files	Patch %	Lines
integration/test_collection_openai.py	41.25%	84 Missing ⚠️
weaviate/collections/batch/base.py	83.87%	45 Missing ⚠️
integration/test_collection_rerank.py	32.69%	35 Missing ⚠️
integration/test_batch_v4.py	95.00%	14 Missing ⚠️
integration_v3/test_backup_v4.py	92.76%	11 Missing ⚠️
mock_tests/test_collection.py	85.71%	10 Missing ⚠️
weaviate/collections/aggregations/base.py	94.57%	9 Missing ⚠️
integration/test_collection.py	99.05%	8 Missing ⚠️
weaviate/collections/batch/batch_wrapper.py	86.20%	8 Missing ⚠️
integration/conftest.py	88.52%	7 Missing ⚠️
... and 12 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #843      +/-   ##
==========================================
+ Coverage   93.48%   95.50%   +2.02%     
==========================================
  Files          70      167      +97     
  Lines        8625    17089    +8464     
==========================================
+ Hits         8063    16321    +8258     
- Misses        562      768     +206     

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