Add support for --extension-name in new extension creation

[joelonsql]

• Added an optional --extension-name parameter to the New struct in cargo-pgrx.
• Updated the execute method to use the provided extension_name if available, defaulting to the crate name otherwise.
• Modified create_crate_template to pass extension_name to template functions.
• Adjusted create_control_file, create_lib_rs, and template files to use extension_name instead of name.
• Ensured that the control file and related files reflect the custom extension name.

This change allows users to specify a different PostgreSQL extension name when creating a new extension crate using pgrx new.