e2e: add test specs for `oras manifest push` command

[qweeah]

Happy paths:

• should push a manifest from stdin
• should push a manifest from a file with media type flag
• should push a manifest from a file without media type flag
• should push a manifest from a file and output the descriptor
• progress status should be disabled when --descriptor is provided
• should show preview hint in help doc

Bad paths:

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

[yuehaoliang]

Happy path:

• Should print prettified descriptor when --descriptor --pretty is applied.

Bad path:

• Should fail if the file is not in valid json format, and --media-type is not specified.
• Should fail if the file is in json format but does not contains mediaType field, and --media-type is not specified.