e2e: add test specs for `oras push` command

[qweeah]

Happy paths:

• should push files with customized media types to remote registry
• should push files with customized media types to remote registry and export the packed manifest
• should push files to remote registry with customized config file
• should push files to remote registry with customized config file and config type
• should push files to remote registry with customized manifest annotation
• should push files to remote registry with customized file annotation
• should show correct push status

Bad paths:

• should fail directly if not logged in
• should fail if no target reference provided
• should fail if no file reference provided