Enhance manifet fetch

[qweeah]

1. Refactor the implementation with oras.Fetch and oras.FetchBytes in oras-go
2. Add below UX

# Fetch
# add output flag
oras manifest fetch <name> --pretty > <path>                     # manifest prettified to file via stdout
oras manifest fetch <name> --output <path> --pretty              # manifest raw to file
oras manifest fetch <name> --output - --pretty > <path>          # manifest prettified to file
oras manifest fetch <name> --output <path> --pretty --descriptor # descriptor prettified
# --output - cannot be used with --descriptor at the same time

[shizhMSFT]

@yuehaoliang-microsoft Could you help this issue?

[yuehaoliang]

I'll leverage new methods in oras-go to improve oras manifest fetch, and support new flags.

[yuehaoliang]

@shizhMSFT Should oras manifest fetch <name> --output <path> --pretty save a prettified content to the file? If not, --pretty is meaningless when used with oras manifest fetch <name> --output <path>, and then should it through an error in this case?
@lizMSFT I think it also applies to blob fetch.

[shizhMSFT]

Should oras manifest fetch --output --pretty save a prettified content to the file?

No. --pretty should only work on stdout. If --pretty is specified with --output, the --pretty option is ignored.

[yuehaoliang]

Should oras manifest fetch --output --pretty save a prettified content to the file?

No. --pretty should only work on stdout. If --pretty is specified with --output, the --pretty option is ignored.

Thank you for clarifying. I see, --pretty should be just ignored when used with oras manifest fetch <name> --output <path>.