Support writing bundles to stdout

[ralphbean]

Description

I was messing around with using cosign to write multiple bundles to a jsonl. I'm not entirely sure if that's a good pattern or useful thing yet, but a nice way to support it would be if cosign allowed writing bundles to stdout, like:

cosign attest-blob --predicate ./predicate.json --bundle -  ./my-blob.bin

Then, I could redirect that to append to a jsonl file or pipe it to other things for other unknown use cases.

Perhaps, --bundle should default to - to assume stdout for attesting. (However, it probably shouldn't default to - for stdin for verifying, b/c people would get confused when they omitted it and the program just hung waiting for stdin input.)