Support String Keys in SummingMergeTree Maps

Why it matters

This feature allows users to define maps with string keys in SummingMergeTree tables, enabling aggregation of data where keys are strings rather than only numeric types. It enhances the flexibility of SummingMergeTree storage engine to handle more complex and diverse data representations.

How to use it

Define a SummingMergeTree table with a map column having string keys, for example:

CREATE TABLE example (
  id UInt32,
  map_col Map(String, UInt64)
) ENGINE = SummingMergeTree()
ORDER BY id;

Insert and query the table as usual; aggregation will now correctly merge rows based on string-keyed maps.