Add GitHub Actions workflow for CI and some fixes

[higebu]

I'd like to add a GitHub Actions workflow for CI, because Travis CI link is broken.

• Update go version to 1.20 in go.mod
• Add github.com/ishidawataru/sctp to go.mod
• Fix tests with tls
• Use go:build style build constraints
• Use context pkg instead of golang.org/x/net/context
• Add GitHub Actions workflow for CI
• Fix panic getLocalAddresses() on sctp

[higebu]

You can check the result of the action here: https://github.com/higebu/go-diameter/actions/runs/5196539785/jobs/9370348449

[fiorix]

Thank you!