The experimental writer need pandas dependency

[fvaleye]

Environment

Delta-rs version: python-v0.5.7

Binding: Python

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Bug

What happened:
When installing the deltalake python package without pandas as optional dependency.

The import fails in the writer.py file since it required the pandas dependency:

site-packages/deltalake/writer.py:8: in <module>
    import pandas as pd
E   ModuleNotFoundError: No module named 'pandas'

What you expected to happen:
Have everything ready to work without requiring to install pandas if we are not using the writer capability.

Possible solutions:

• Create a dedicated write_deltalake_from_pandas method to avoid checking the instance of the data as dataframe
• Add only pandas for types checking like this
• Check if pandas is installed with a bool when importing and add this check before this check