add "init templates" cmd, used to list templates in format

[healthjyk]

What type of PR is this?

feature

What this PR does / why we need it:

list init templates from local or online, support json format. Hence, user can parse the specific elements of the templates.

Which issue(s) this PR fixes:

Fixes #265

Special notes for your reviewer:

If the cmd failed, the output is not in json format, even if "--output=json" is specified.

Does this PR introduce a user-facing change?

None

Additional documentation e.g., design docs, usage docs, etc.:

kusion init templates command helps you get the templates which are used to generate a scaffolding KCL project.
Here are the examples:
# Get name and description of internal templates
  kusion init templates
# Get templates from specific templates location, and output in json format
  kusion init templates https://github.com/<user>/<repo> --online=true --output=json

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[healthjyk]

I have read the CLA Document and I hereby sign the CLA

[SparkYuan]

@healthjyk please add an example output for this command

[healthjyk]

@healthjyk please add an example output for this command

an successful request is like:

[
    {
        "dir": "/${userName}/.kusionup/current/templates/external/code-city",
        "name": "code-city",
        "projectName": "code-city",
        "description": "Code City metaphor for visualizing Go source code in 3D.",
        "quickstart": "kusion compile main.k -Y ci-test/settings.yaml",
        "projectFields": [
            {
                "name": "AppName",
                "description": "App name. Default to 'gocity'.",
                "type": "string",
                "default": "gocity"
            },
            {
                "name": "ContainerPort",
                "description": "The number of port to expose on the container's IP address.",
                "type": "int",
                "default": 4000
            },
            {
                "name": "ServicePort",
                "description": "The port that will be exposed by this service.",
                "type": "int",
                "default": 4000
            }
        ],
        "stacks": [
            {
                "name": "dev",
                "fields": [
                    {
                        "name": "Image",
                        "description": "Docker image address of gocity. Default to 'yuanhao1223/gocity:latest'.",
                        "type": "string",
                        "default": "howieyuen/gocity:latest"
                    }
                ]
            }
        ]
    }
]

[coveralls]

Pull Request Test Coverage Report for Build 4915161384

• 102 of 153 (66.67%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.08%) to 71.913%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/cmd/init/init.go	18	27	66.67%
pkg/cmd/init/options.go	84	126	66.67%

Files with Coverage Reduction	New Missed Lines	%
pkg/cmd/init/options.go	2	69.9%

Totals
Change from base Build 4913825402:	0.08%
Covered Lines:	4857
Relevant Lines:	6754

---

💛 - Coveralls

[amyXia1994]

The error message should output to stderr, but now the wrong git repo url will result in an error ouput in stdout. :
kusion init templates https://github.com/KusionStack/no-exist --online=true -o json
❌ no templates

kusion init templates KusionStack/kusion-templates.git --online=true -o json
❌ template 'kusionstack/kusion-templates.git' not found

kusion init templates https://github.com/kusion-templates.git --online=true -o json
❌ failed to retrieve git folder: invalid Git URL

[amyXia1994]

I'll merge this PR. please resolve the stderr and subcommand name problems in a separate PR.