Allow creation of summary stats by passing in a dataframe

[paddymul]

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Developer ergonomics (defaults, error messages)

Enhancement Description

It should be possible to call Buckaroo with an argument like
Buckaroo(df, extra_summary_stats=summary_stats_df) where summary_stats_df is a dataframe that gets turned into summary stats rows (which are available for referencing in styling)

Columns should be in common with the main dataframe.

Necessary for #335

Pseudo Code Implementation

No response

Prior Art

N/A