fix: add plain-http option for login cmd

[chlins]

This pull request includes several changes to the cmd/login.go and pkg/backend files to replace the Insecure flag with a new PlainHTTP flag for allowing HTTP connections to the registry. Additionally, the backend login function has been modified to accept options.

Changes to cmd/login.go:

• Updated the login command description to reflect the new PlainHTTP flag.
• Replaced the Insecure flag with the PlainHTTP flag in the command initialization.
• Modified the runLogin function to handle the new PlainHTTP flag using backend options.

Changes to pkg/backend:

• Updated the Login method signature in the Backend interface to accept options instead of the insecure boolean.
• Modified the Login method implementation to apply the new options and handle the PlainHTTP flag accordingly. [1] [2]

Changes to pkg/config/login.go:

• Replaced the Insecure field with the PlainHTTP field in the Login struct and its constructor.

[gaius-qi]

LGTM