Allow more checksum algorithms for each file

At the moment, [only one algorithm can be used at a time](https://opensource.axo.dev/cargo-dist/book/artifacts/checksums.html). Providing more checksum files can be useful when you want to provide one using a modern algo such as BLAKE2, along with a widely available one, such as SHA-256.

Start with the checksum documentation linked in the issue and trace how cargo-dist currently selects one checksum algorithm per file. Define the configuration and output behavior needed to provide multiple checksum files, including BLAKE2 and SHA-256, then verify the generated artifacts and documentation describe the supported combinations.

Written by the indexing model from the issue text.