Fixed the content type in requests with external data

[mynameisscr]

Added an explicit indication of the text/plain content type with UTF-8 encoding for an SQL query in MultipartEntityBuilder, otherwise the text/plain content type with ISO_8859_1 encoding is used and Latin letters turn into'??????'

[github-actions]

Benchmark                           (client)  (statement)   Mode  Cnt     Score     Error  Units
Basic.insertOneRandomNumber  clickhouse-jdbc       normal  thrpt   20   285.047 ±  29.502  ops/s
Basic.insertOneRandomNumber  clickhouse-jdbc     prepared  thrpt   20   274.140 ±  28.611  ops/s
Basic.selectOneRandomNumber  clickhouse-jdbc       normal  thrpt   20  1925.755 ± 251.761  ops/s
Basic.selectOneRandomNumber  clickhouse-jdbc     prepared  thrpt   20  1825.250 ± 242.755  ops/s

[zhicwu]

Thanks for your contribution @MikhailMel. The change has been merged.