Add crc32c as checksum backend

[kreczko]

CRC32C hardware implementation in python is provided by the crc32c package.

The package supports partial checksums and should be easy to integrate.

Steps:

1. move int_to_hex and hex_to_int outside the Checksum class
2. add implementation src/xrdsum/checksums/crc32c.py
3. add "crc32c" to AVAILABLE_CHECKSUM_TYPES