Make `incus top` output configurable through options

[presztak]

Adds support for following options:

• --format - possibility to define output format
• --columns - possibility to define column layout
• --refresh - possibility to configure the refresh delay

Closes: #1028

[stgraber]

Looks good, just did a few minor edits:

• Re-titled and fleshed out main commit message
• Added validation for --format so it only allows table and compact (along with their extra options) as otherwise --format=json was considered valid
• Added validation for --refresh to make the minimum refresh rate 10s (matches server side minimum for metrics)
• Updated a comment (was mentioning making --refresh a flag, now it has been)