Pairwise string comparison

[ADBond]

Following Splink#2517 and Splink#2546, add Clickhouse-specific versions of PairwiseStringDistanceFunctionLevel and PairwiseStringDistanceFunctionAtThresholds.

These could not work using just the dialect markers introduced in the latter PR, as the SQL in Splink works on the assumption that the lambda is passed as the second argument to functions. However, in Clickhouse it must be passed as the first argument, and the parser will fail if it is passed elsewhere. This rules out options such as defining a udf with the arguments switched (as the parser will still fail). Rather than using some more involved string manipulation, we simply re-implement the SQL in a way that is appropriate for Clickhouse dialect.

This actually turned out to be doubly useful as it is also not directly possible to unnest lists by a single level in Clickhouse, requiring a workaround (although tbf this could have still been achieved with a udf).

Mainly to facilitate testing this also allows ClickhouseAPI to correctly register pandas columns that are arrays of strings. For now any other array columns will be coërced to arrays of strings (I think, though haven't tested).
For chdb this is currently less straightforward as we rely on its native SELECT * FROM Python(input) rather than doing any manual processing, and this does not presently recognise array types.