Unable to test plugins with TLS channel on Windows

[marcintao]

Description
When starting an example plugin (rand) on Windows with TLS enabled, error prevents plugin from working. Plugin complains that system root certificates are unavailable on Windows.

Steps to reproduce

1. build example plugin for windows

cd examples/snap-plugin-collector-rand
GOOS=windows GOARCH=386 go build

2. launch example plugin with TLS enabled

snap-plugin-collector-rand.exe "{\"CertPath\":\"libtest-srv.crt\",\"KeyPath\":\"libtest-srv.key\"}

Observed outcome
Plugin throws an error:
panic: unable to read root CAs: crypto/x509: system root pool is not available on Windows

Expected outcome
Plugin starts without issues.

Remarks
I understand that golang doesn't have complete support for root certificates on Windows, according to issue golang/go#16736.
It would be better to add support for explicitly specified root certificates for all plugins based on this library, e.g.: RootCertPaths: \"libtest-CA.crt;clients-use-this.crt\".