fix(clickhouse sink): add support for default columns (#24692)

* fix(clickhouse sink): add support for default columns

* refactor: use serde_json::Deserializer::from_str to parse JSON response

* refactor: streamline schema fetching and skip UTF-8 allocation

* refactor(clickhouse sink): extract column kind string literals into constants

* refactor(clickhouse sink): use indoc! for multiline test strings

* fix(clickhouse sink): use proper Timestamp value for DEFAULT column in test

The Arrow serializer expects a typed Timestamp value, not a plain string,
for DateTime columns. Using a string caused a chrono parse error that
prevented the batch from being sent.

* chore: fmt

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Co-authored-by: Pavlos Rontidis <pavlos.rontidis@gmail.com>