fix: Add license file to v3 and v3/newrelic

[tydavis]

Adds license file to v3/ and v3/newrelic/ subdirectories to make the
repository compatible with pkg.go.dev (in particular,
https://pkg.go.dev/github.com/google/licensecheck).

Resolves #169 #169

[CLAassistant]

All committers have signed the CLA.

[tydavis]

@jthurman42 - bringing this to your attention

[tydavis]

I'm not sure why the two integration tests failed?
https://github.com/newrelic/go-agent/pull/254/checks?check_run_id=1573077495
https://github.com/newrelic/go-agent/pull/254/checks?check_run_id=1573077118

88
	github.com/newrelic/go-agent/v3/integrations/nrawssdk-v2.test imports
89
	github.com/aws/aws-sdk-go-v2/service/dynamodb imports
90
	github.com/awslabs/smithy-go/httpbinding: package provided by github.com/awslabs/smithy-go at latest version v0.4.0 but not at required version v0.4.1-0.20201216214517-20e212c92831 

and

25
	/opt/hostedtoolcache/go/1.13.15/x64/src/github.com/aws/aws-sdk-go-v2/private/protocol/rest (from $GOROOT)
26
	/home/runner/work/go-agent/go-agent/src/github.com/aws/aws-sdk-go-v2/private/protocol/rest (from $GOPATH)
27
package github.com/aws/aws-sdk-go-v2/aws/external: cannot find package "github.com/aws/aws-sdk-go-v2/aws/external" in any of:
28
	/opt/hostedtoolcache/go/1.13.15/x64/src/github.com/aws/aws-sdk-go-v2/aws/external (from $GOROOT)
29
	/home/runner/work/go-agent/go-agent/src/github.com/aws/aws-sdk-go-v2/aws/external (from $GOPATH)

Look to be errors with the versioned aws-sdk libraries?

[tydavis]

Are there any concerns or blockers to merging this issue? I'm unsure how to resolve the AWS SDK build test issues.

[Clivern]

It would be nice to merge this since go pkg docs not showing for v3 https://pkg.go.dev/github.com/newrelic/go-agent/v3/integrations/logcontext/nrlogrusplugin

[tydavis]

Bumping this again - @jthurman42 @jodeev any issues with merging?

[RichVanderwal]

Thanks for your contribution, @tydavis ! Merging to develop now and releasing soon.