feat(cmd): add import command

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Description

This MR creates a new CLI command under variables which helps the user to "import" the CI/CD variables from JSON files as well as STDIN

Affected files:

1. internal\commands\variable\import\import.go (new file)
2. internal\commands\variable\import\import_test.go (new file)
3. internal\commands\variable\variable.go

Related Issues

Resolves #7952

How has this been tested?

1. I created a glab.exe by executing the main.go and ran the CLI command pointing to my personal fork and verified the existence of the variables under CI/CD section

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap