e2e: add test specs for oras manifest fetch command

[qweeah]

Happy paths:
Before 0.15.0

• should fetch a manifest list via a tag
• should fetch a manifest list via a digest
• should fetch the descriptor of a manifest via a tag
• should fetch the descriptor of a manifest via a digest
• should fetch a manifest with a target platform from a manifest list
• should fetch the descriptor of a manifest with a target platform
• should fetch a manifest with platform validation
• should fetch the descriptor of a manifest with platform validation
• should fetch a manifest with customized media type
• should show experimental preview hint in help doc

Before 0.16.0

• should fetch a manifest to stdout and prettify
• should fetch a manifest to file and output the descriptor to stdout at the same time
• should fetch a manifest to file and output the prettified descriptor to stdout

Bad paths:

• should fail directly if not logged in
• should fail if no artifact reference provided
• should fail fetching a not existed image
• should fail fetching if customized media type is provided but non matched manifest found