Open source a generic http client module to garr

[Permagate]

• [x] Copy module to garr.
• [x] Adapt changes to open source.
  • [x] Add apache license.
  • [x] Update module name to a generic httpclient.
  • [x] Update some imports to go.linecorp.com/garr.
  • [x] Update tests to use only go standard test package.
• [ ] Fix data race.
• [ ] Create readme and examples.