Move httpclient to ecs-agent/ and minor refactor

[danehlim]

Summary

Move httpclient package from agent module to ecs-agent module and also perform minor refactor of the same. This is required for future changes in ecs-agent module that depend on this package. If the httpclient package is not moved, then in the future a dependency cycle could be introduced in which the ecs-agent module depends on agent module while the agent module already depends on ecs-agent module.

Implementation details

• Moves files from agent/httpclient/ to ecs-agent/httpclient/
• Update dependent imports accordingly
• Minor refactor to ecs-agent/httpclient/httpclient.go
  • ecsRoundTripper struct now has 2 new struct fields agentVersion and osTypeand related New function now takes in Agent version and OS type as arguments. This is so that we avoid making assumptions about the underlying Agent (i.e., this way, we do not need to take a dependency on the agent/version and agent/config packages)
  • Change the TLS handshake timeout used to be a constant defaultTLSHandshakeTimeout instead of a hardcoded value
  • Add code comment to provide additional context/reference on where defaultDialTimeout, defaultDialKeepalive defaultTLSHandshakeTimeout constant values come from
  • Use ecsRoundTripper transport's DialContext field in favor of its Dial field, since http.Transport.Dial is deprecated

Testing

Unit, integration, and functional tests.

New tests cover the changes: no, but existing tests updated

Description for the changelog

Move httpclient to ecs-agent/ and minor refactor

Does this PR include breaking model changes? If so, Have you added transformation functions?
No

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.