Support context in providers (#519)

CHANGELOG.md

oauthproxy.go

oauthproxy_test.go

pkg/requests/requests.go

pkg/requests/requests_test.go

providers/azure.go

providers/azure_test.go

providers/bitbucket.go

providers/bitbucket_test.go

providers/digitalocean.go

providers/digitalocean_test.go

providers/facebook.go

providers/github.go

providers/github_test.go

providers/gitlab.go

providers/gitlab_test.go

providers/google.go

providers/google_test.go

providers/internal_util.go

providers/internal_util_test.go