add polars backend to get_frequency_summary - first draft

[GTimothee]

Here is a first draft.

It is working as is but I have two concerns:

_get_pandas_frequency is difficult to convert because there is currently no idx.inferred_freq equivalent in polars.
The only solution I can think of is to write an equivalent of inferred_freq pandas implementation, but I am not sure if it is worth the time.

I don't know what unit is expected for freq_median_timedelta.
Again, there is no timedelta in polars so I create it from _freq_median_seconds and put the unit to seconds.
But the output is probably not consistent with the pandas implementation then ?

Linked to #77

[mdancho84]

I don't think at this point the polars inferred freq should be attempted. I agree this is a big undertaking given our resources. We are better suited to build out the finance module.