set camera frame rate in exportAnimatedCamera

[servantftransperfect]

This pull request introduces support for specifying the camera frame rate when exporting animated cameras. The changes add a new frameRate parameter to both the command-line interface and internal methods, allowing users to define the number of frames per second for camera animation exports.

Animated Camera Export Improvements

• Added a frameRate parameter to the ExportAnimatedCamera node, allowing users to define the camera's frames per second via the UI.
• Updated the initAnimatedCamera method in AlembicExporter to accept a frameRate argument, and adjusted the time sampling logic to use the specified frame rate instead of a fixed value.
• Added a frameRate variable to the main export tool and exposed it as a command-line option, enabling users to set the frame rate when running the export tool.
• Modified the call to initAnimatedCamera in the export tool to pass the user-defined frameRate value, ensuring the exported animation uses the correct timing.