Support format options for buf curl --data

[joshcarp]

The buf convert --from and --to flags both support specifying the format with #format=[json|bin] or file extensions. Supporting this for buf curl would also be nice and consistent with the behaviour of other buf flags:

 echo '{"name": "Alice", "age": 30}' | buf curl <url> --data=-#format=json
cat file.bin | buf curl <url> --data=-#format=bin