Description

This PR adds support for nested options in CLI configurations, particularly
when using pointer-to-struct fields. It addresses an issue where the CLI would
panic with panic: Unsupported option type: ptr or panic: Unsupported option type: struct
when attempting to set values on fields within a nil pointer struct.

Example Use Case (my use case).

I was trying to implement nested configuration options with pointers for a clean optional field pattern:

type DatabaseConfig struct {
    Host     string `doc:"Database host"`
    Port     int    `doc:"Database port" default:"5432"`
    Username string `doc:"Database username"`
}

type AppConfig struct {
    Debug bool            `doc:"Enable debug mode"`
    DB    *DatabaseConfig `doc:"Database configuration"` // Here both ptr or direct would have been acceptable.
}

When attempting to use this pattern with CLI flags like
--db.host localhost --db.port 5432,
I encountered a panic error: panic: Unsupported option type: struct.
The fix in this PR allows for properly handling these nested pointer-to-struct
configurations, so users can effectively organize related options.

Changes

• Improved handling of nested fields by properly initializing nil pointers
  before setting values
• Enhanced error handling with more descriptive messages and proper error
  wrapping

I tried to split every major change in its own commit, btw

Testing

Added a new test case TestCLINestedOptions that verifies both direct struct
fields and pointer-to-struct fields work correctly with CLI arguments and
environment variables.

Oh ! And thanks for your awesome project 💯