net-mgmt/netdata: add GOPLUGIN option (default off)...

...to create a run-time dependency on net-mgmt/netdata-go.

PR: 244304
Submitted by: Dries Michiels <driesm.michiels@gmail.com>
Reviewed by: daniel.engberg.lists@pyret.net
Approved by: maintainer timeout (mmohki@, 42 days)