feat(flux get): add --output flag, yaml only

[mtfurlan]

Copied the flag syntax from kubectl --output so it can be extended later
without breaking changes.

It's not integrated terribly well, but the next step would be to
abstract the printing so we can switch out printers more cleanly and
also pick which information is rendered for each printing format, and I
do not know go or this codebase well enough to do that yet.

Signed-off-by: Mara Furland mara@fur.land

[mtfurlan]

zero rows print nothing—fine if deliberate.

Maybe a warning on stderr would be nice, but if you're doing all resources it's going to be quite hard to check.

YamlPrinter assumes len(row) >= len(header); a short row would panic

It doesn't panic, but it does assume that header[i] maps to row[i], so if headers is short, then it just won't add the row
You did cause me to notice --no-headers though, so that's fixed

I'm still not sure how to fix the boolean output though EDIT: I did it!

[stefanprodan]

For get commands the --output should return a List type like kubectl does, having the table printed in yaml format is not something I would consider, we need to return all objects in a list.

[mtfurlan]

I don't disagree, but the requests for this have existed for a few years and this is still a lot better than the current state.
Could name the output yamltable so we could do real yaml later without any breaking change?

If you can help me with a plan I can spend a little more time trying to make it do more real yaml output.
The only idea I have right now is to modify summarisable to include a getRaw(i int) any and then pass the summarisable to the printer interface, but I don't like any and I don't know go typing well enough to do this on my own.