Add flag for pretty-printing discovery JSON

[caxu-rh]

This PR addresses #9.

• Open to alternative naming for the long or short names of the flag. I know of this feature as "pretty-printing" but I suppose it could also be called "formatted" JSON - but I tend to associate --format with accepting a template, e.g. {{ .Foo }} when I want a program (kubectl get, podman inspect) to give me some portion of the JSON output (maybe that could be useful too, but a separate feature).
• This defaults to sticking with the one-line, unformatted output. We could also switch this to default to multiline, formatted output, and have a flag to choose unformatted (maybe a -r/--raw-output flag instead or something similar).
• We add a new boolean flag on NewManifestJSONProcessorFn. I considered making this function take a new kind of config/opts struct instead (for future expansion), but I think it could wait for later if/when more options are possible.
• We currently format with indentation of 4 spaces, not sure if this should be 2, or if it should also be configurable.

[komish]

@caxu-rh you can just make it pretty by default, and remove the flag altogether. I'm of the "jq exists" mindset, where if someone needed to go from raw --> pretty or vice versa, they could pipe it out to jq.

That said, pretty printing is a reasonable default, but I don't think you need to jump through the hoops to offer both here.

[EDIT] Actually, I'll leave having this be configurable up to your discretion. If you want to leave it, pretty as a default makes sense to me (so make the flag --raw instead)

[komish]

Oh, and if you end up leaving it - feel free to add a config struct as you see fit.

[caxu-rh]

Adjusted to flip the default behavior, forgot to add a config struct - expect another follow-up change.

[caxu-rh]

• Renamed the option to --compact / -c for anyone wanting to keep the compacted output. I figure it is still helpful to keep around in case someone is running this in e.g. some CI flow where some external tools might not be available. I think the naming "compact" is more accurate than "raw" (implies that non-raw output is going through some kind of post-processing, which is not the case) - and I realized that jq's --compact-output option is closer to what we are doing here (--raw-output in jq is also omitting escape chars).
• Added a new struct type NewManifestJSONProcessorFnOptions - open to naming adjustments. The options in this struct are unique for making a ProcessingFunction that outputs a manifest JSON (not generic to ProcessingFunctions in general). To keep things simple for now, this change includes only the struct (avoiding adding e.g. WithCompactOutput() options-style functions).

[komish]

/lgtm

[exe-prow-github-app]

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