Required by default, granular opt-in/out

[abhinav]

There are frequent cases where "field is required" is the default, and optional is the exception.
e.g. there are a fair number of instances of // optional [desc] in the Go source itself.

There should be a way to opt into "required by default" at a package level.
This wouldn't be a global default for this because it would have far too many false positives.

With package-level opt-in, we'll inevitably need struct-level opt-out, so this issue is also for granular required/optional opt-in for structs or packages.