[Resources.Container] Rename Container detector namespace

[joegoldman2]

Contributes to #1610.

Changes

• Renames OpenTelemetry.ResourceDetectors.Container to OpenTelemetry.Resources.Container
• Makes detector internal
• Adds AddContainerDetector extension method for ResourceBuilder

For significant contributions please make sure you have completed the following items:

• [x] Appropriate CHANGELOG.md updated for non-trivial changes
• [ ] Design discussion issue #
• [ ] Changes in public API reviewed