nix-prefetch-url: support SRI hashes directly

[eclairevoyant]

Is your feature request related to a problem? Please describe.
Currently, using SRI hashes with nix-prefetch-url require an invocation such as:

nix-prefetch-url <url> --type sha256 | xargs nix hash convert --hash-algo sha256

Describe the solution you'd like
It'd be far more convenient to have a flag to output SRI hashes directly, e.g.

nix-prefetch-url <url> --type sha256 --as-sri

Or alternatively, use SRI hashes by default, since they're the preferred type in nixpkgs anyway.

Describe alternatives you've considered
see above

Additional context

Priorities

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[Chatchaton]

having a subcommand nix prefetch-url with --as-sri as the default behavior would be nice

[edolstra]

nix store prefetch-file prints an SRI hash, maybe that works for you?