Update sdk go modules

[alvarocabanas]

• Upgraded github.com/newrelic/infra-integrations-sdk to v3.6.7
• Switched to go modules
• Upgraded pipeline to go 1.16
• Replaced gometalinter with golangci-lint
• Added integration tests

[varas]

good job! I added some comments, but just the v3/4 is a real concern