feat: endpoints credentials handling

lib/config/env.js

lib/config/index.js

lib/datasource/github.js

lib/datasource/npm.js

lib/platform/github/gh-got-wrapper.js

lib/platform/github/index.js

lib/platform/gitlab/gl-got-wrapper.js

lib/platform/gitlab/index.js

lib/platform/vsts/index.js

lib/platform/vsts/vsts-got-wrapper.js

lib/platform/vsts/vsts-helper.js

lib/util/endpoints.js

lib/workers/global/index.js

lib/workers/pr/changelog/index.js

lib/workers/pr/changelog/source-github.js

test/config/__snapshots__/env.spec.js.snap

test/config/env.spec.js

test/config/index.spec.js

test/platform/github/index.spec.js

test/platform/gitlab/index.spec.js