actions: download: Add support for integrity checks

[ivy]

When downloading files from remote URLs, it would be helpful to have some sort of integrity check to ensure the file wasn't corrupted or tampered with. To start, I'd consider adding a new key, integrity, accepting a hash to specify the algorithm and hex-encoded checksum:

- action: download
  url: http://example.domain/path/filename.ext
  name: firmware
  integrity: 
    hash: sha256-5ecf8d2cc410094e8b82dd0bc178a57f3aa1e80916689beb00fe56148b1b1256

My reasoning for the nested struct/table is that it leaves this open for extension later, possibly adding support for GPG signatures.