feat: Use cobra.Command.OutOrStdout method for output

[TerryHowe]

What this PR does / why we need it:

This contains the command refactor I did in another PR.

With this change, the output of a command can be set to something other than stdout. For example capture the output of a command:

    var stringWriter strings.Builder
    cmd := root.New()
    cmd.SetArgs([]string{"repo", "ls", "mcr.microsoft.com"})
    cmd.SetOutput(&stringWriter)
    _ := cmd.Execute()
    fmt.Println(stringWriter.String())

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.48936% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 82.04%. Comparing base (f1d319f) to head (fe953b1).

Files	Patch %	Lines
cmd/oras/root/login.go	66.66%	1 Missing and 2 partials :warning:
cmd/oras/root/discover.go	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1288 +/- ## ========================================== + Coverage 81.94% 82.04% +0.09% ========================================== Files 83 83 Lines 4005 4016 +11 ========================================== + Hits 3282 3295 +13 + Misses 500 498 -2 Partials 223 223 ```

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[qweeah]

Need to confirm which output should be covered in this PR.

The proposed change in this PR doesn't cover all output of ORAS. E.g. the output of oras push/attach --format xxx which is designed to be used for automation. (There are also undergoing efforts for other commands, see #638) We have a draft doc https://hackmd.io/@shizh/HyFZkEcca to categorize all types of output in ORAS.

@TerryHowe Looks like you are trying to do automation and the target output to be changed is metadata and raw output?

[FeynmanZhou]

Thanks @TerryHowe . This is a big PR. From users' perspective, I am curious on what's the impact to ORAS CLI output. There is not an issue associated with this PR to describe the problem or enhancement.

[TerryHowe]

This change has no effect on the format of the output and from my understanding no effect on CLI output. The change is really just for go scripting.

• This change uses Fprintln instead of Println so we can pass a writer.
• For the CLI cmd.OutOrStdout() will always return os.Stdout so no change in behavior

[TerryHowe]

The goal is to allow oras go scripting to be able to capture output from commands without messing with stdout

[qweeah]

The goal is to allow oras go scripting to be able to capture output from commands without messing with stdout

The output of --format won't go into STDOUT, e.g. localhost:5000/test@sha256:16ce5d4fb98496ec805e6b2401213598e710dbd936fb58cb7a325d2582924694 cannot captured by the go script.

> oras version
Version:        1.2.0-beta.1
Go version:     go1.21.6
Git commit:     9ffdb3eec60b969d842af1a9e699202e0827fa01
Git tree state: clean
> oras push localhost:5000/test:format --format {{.Ref}}
✓ Uploaded  application/vnd.oci.empty.v1+json                                                      2/2  B 100.00%   37ms
  └─ sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a
✓ Uploaded  application/vnd.oci.image.manifest.v1+json                                         535/535  B 100.00%     0s
  └─ sha256:16ce5d4fb98496ec805e6b2401213598e710dbd936fb58cb7a325d2582924694
localhost:5000/test@sha256:16ce5d4fb98496ec805e6b2401213598e710dbd936fb58cb7a325d2582924694

[TerryHowe]

The output of --format won't go into STDOUT, e.g. localhost:5000/test@sha256:16ce5d4fb98496ec805e6b2401213598e710dbd936fb58cb7a325d2582924694 cannot captured by the go script.

I'd sooner handle format as a follow up.