Add forc plugins command flag for outputting JSON or TOML for easier machine readability

[mitchmindtree]

This would potentially make it easier for downstream tooling to integrate with forc plugins.

Follow-up to #1199.

[kayagokalp]

Is this still relevant? If so maybe we can clarify this a little bit and I can take a look into this one (if it is ok).

The part that I would like to clarify is the content of JSON or TOML output. Is it going to include:

• Name of the plugin
• Path of the plugin
• Description of the plugin
• (all above)

I can see that there are separate issues (#1214, #1215) for the printing path of the plugin and the description of the plugins with a flag. I think we have two main possibilities here:

1. We can include all mentioned information (paths, names, descriptions) by default to the JSON output
2. We can include the information if the corresponding flag is present

I think option 2 is closer to the default behavior but option 1 is also logical since the output is going to be parsed the output can include everything by default.

[mitchmindtree]

Sorry about the delayed response to this! Caught me while I was out-of-action :)

Is this still relevant?

I think this is still relevant, but I think the issues @JoshuaBatty picked are probably more useful in the meantime as we don't have any direct use-cases for this just yet.

I think we have two main possibilities here:

I have no major preference on either approach, you make a good case for both! When we get around to this, we could possibly find a similar tool and see what they do, or alternatively just choose the approach that integrates into our existing code in the easiest manner.

I think this is also vaguely related to #885, where we want to offer a serializable, structured alternative to human-readable content that gets written to stdout.

[mitchmindtree]

Was just reading some cargo docs and came across this section which describes how they support JSON output in a bit more detail: https://doc.rust-lang.org/cargo/reference/external-tools.html#json-messages