rpk: add interactive flag to generate grafana-dashboard

[r-vasquez]

Fixes #9485

Example:

$ rpk generate grafana-dashboard --interactive

Backports Required

• none - issue does not exist in previous branches

No release notes since this is a new feature and the release notes will be in #9662

Release Notes

• none

[twmb]

Can we default to interactive if no flag is used, rather than requiring an --interactive flag?

[r-vasquez]

That would be a similar behavior to the log-level change command, however, here it will break users that expect an output when running rpk generate grafana-dashboard without a --dashboard flag.

Also, the current --dashboard flag defaults to a specific dashboard so they will get the operations dashboard if no flags are provided.

[r-vasquez]

Won't do. --interactive is not discoverable enough and does not bring additional value to the command space.