feat(cli): manifest command with add flag

[kriscoleman]

feat(cli): manifest command with add flag

Add a new manifest command to the CLI that allows users to manage their flag manifest file directly from the command line. This will provide a more streamlined workflow for flag management without requiring manual file editing.

Starting with the add sub-command to add new flags to the manifest file, with plans to expand to other manifest management operations in the future.

Acceptance Criteria

• Implement openfeature manifest add command
• Command requires flag name and default value as mandatory parameters
• Command defaults type to "boolean" but allows override via optional type parameter
• Command accepts optional description parameter
• Command validates flag configuration before adding to manifest
• Use temp file pattern for safe I/O operations:
  • Write updated manifest to temporary file
  • Complete all I/O operations on temp file
  • Only overwrite original manifest if no errors occur
• Command provides clear success/error feedback
• Command follows existing CLI patterns and conventions
• Add appropriate help text and usage examples showing required vs optional parameters
• Include unit tests for the new functionality

Notes

• Required parameters: name, default value
• Optional parameters: type (defaults to "boolean"), description
• Follow existing temp file pattern used elsewhere in codebase for safe file operations
• This is the first step in a broader manifest management feature set
• Future sub-commands may include: remove, list, update, validate
• Should integrate with existing manifest file format and validation logic

[kriscoleman]

defaulting the type to boolean might lead to developer error, accidentally adding flags for different types than the default. Some ways we might mitigate this if we want to keep it as a default is:

• validation: if user provides a default value that doesn't match the type, we should error
• review: before adding the flag, we can prompt the user with a review and y/n, giving them time to catch errors like flag names

else - we don't default

[georgesdugue]

Unable to assign this issue to myself, but I will be working on it.