chore: Add format type text

[TerryHowe]

What this PR does / why we need it:

Introduce text as default format type for pull, push, manifest fetch and attach commands.

The only change in E2E experience is in the help document. Taking oras attach as an example:

Before

> oras attach -h
...
--format string                               [Experimental] Format output using a custom template:
                                                    'json':         Print in JSON format
                                                    'go-template':  Print output using the given Go template
...

After

> oras attach -h
...
      --format string                               [Experimental] Format output using a custom template:
                                                    'json':         Print in JSON format
                                                    'go-template':  Print output using the given Go template
                                                    'text':         Print in text format (default "text")
...

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.32%. Comparing base (2aa005c) to head (40ca076).

Files	Patch %	Lines
cmd/oras/internal/display/handler.go	62.50%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1397      +/-   ##
==========================================
+ Coverage   85.14%   85.32%   +0.17%     
==========================================
  Files         107      107              
  Lines        3790     3795       +5     
==========================================
+ Hits         3227     3238      +11     
+ Misses        336      333       -3     
+ Partials      227      224       -3     

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[qweeah]

Just to clarify: For this PR, which of the following is matches your expectation:

1. Make the format selection code more readable, or
2. Define the behavior of text output in command help doc. E.g. for v.1.2.0 the help doc of oras attach is

> oras attach -h
...
--format string                               [Experimental] Format output using a custom template:
                                                    'json':         Print in JSON format
                                                    'go-template':  Print output using the given Go template
...

And you are trying to change it to:

> oras attach -h
...
--format string                               [Experimental] Format output using a custom template:
                                                    '':             Print in text format
                                                    'json':         Print in JSON format
                                                    'go-template':  Print output using the given Go template
...

/cc @FeynmanZhou

[qweeah]

Should exclude FormatTypeText from oras discover

[qweeah]

Since this PR unifies default value for format type, I am thinking maybe we should take a step further and add a new function to set default and optional allowed types, e.g.

// Format contains input and parsed options for formatted output flags.
type Format struct {
	FormatFlag   string
	Type         string
	Template     string
	defaultType  *FormatType
	allowedTypes []*FormatType
}

// SetTypes sets the default format type and allowed format types.
func (opts *Format) SetTypes(defaultType *FormatType, allowedTypes ...*FormatType) {
	opts.defaultType = defaultType
	opts.allowedTypes = append(allowedTypes, defaultType)
}

So taking oras attach as an example, code changes in command with default text output would be like:

- opts.AllowedTypes = []*option.FormatType{option.FormatTypeJSON, option.FormatTypeGoTemplate}
+ opts.SetTypes(option.FormatTypeText, option.FormatTypeJSON, option.FormatTypeGoTemplate)

For oras discover, the changes can be:

- 	opts.AllowedTypes = []*option.FormatType{
+	opts.SetType(
		option.FormatTypeTree,
		option.FormatTypeTable,
		option.FormatTypeJSON.WithUsage("Get direct referrers and output in JSON format"),
		option.FormatTypeGoTemplate.WithUsage("Print direct referrers using the given Go template"),
-	}
+	)

[TerryHowe]

/lgtm

Nice, thanks!

[shizhMSFT]

LGTM with suggestions and questions.

[qweeah]

LGTM