Allow Strict Lower Boundary for File Segment Size with Background Data Downloading

Allow to have strict lower boundary for file segment size by downloading remaining data in the background. Minimum size of file segment (if actual file size is bigger) is configured as cache configuration setting boundary_alignment, by default 4Mi. Number of background threads are configured as cache configuration setting background_download_threads, by default 2. Also max_file_segment_size was increased from 8Mi to 32Mi in this PR. #51000 (Kseniia Sumarokova).