Refactor Dependency Proxy controller namespacing

What does this MR do?

As the Dependency Proxy feature has grown, it makes sense to namespace it in it's own module to keep the related code together. This MR moves renames two controllers and moves them into the DependencyProxy module:

Before	After
Groups::DependencyProxyAuthController	Groups::DependencyProxy::AuthController
Groups::DependencyProxyForContainersController	Groups::DependencyProxy::ContainersController

Screenshots or Screencasts (strongly suggested)

N/A

How to setup and validate locally (strongly suggested)

N/a

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #337535