e2e: add test specs for `oras blob fetch` command

[qweeah]

Happy paths:

• should fetch blob content and output to a file
• should fetch blob descriptor
• should fetch prettified blob descriptor
• should fetch blob content to a file with prettified descriptor in stdout
• should fetch blob content and output to stdout
• progress status should be disabled when --output - or --descriptor is provided
• should show preview hint in help doc

Bad paths:

• should fail if no subject reference is provided
• should fail if neither output path nor descriptor flag is provided
• should fail if --output - and --descriptor flag are both provided
• should fail directly if not logged in

[lizMSFT]

Happy paths:

• should fetch blob content and output to a file
• should fetch blob content and output to stdout
• should fetch a blob and output descriptor to stdout
• should fetch a blob and output prettified descriptor to stdout
• should fetch blob content to a file and output prettified descriptor to stdout
• progress status should be disabled when --output - or --descriptor is provided
• should show preview hint in help doc

Bad paths:

• should fail directly if not logged in
• should fail if no subject reference is provided
• should fail if the blob reference is not in the form of <name@digest>
• should fail if the specific blob does not exist
• should fail if neither output path (--output) nor descriptor flag (--descriptor) is provided
• should fail if --output - and --descriptor flag are both provided