add max_num_images setter to C api

[mdwekat]

hi, another small one.

MainExecutorSettings::SetMaxNumImages exists in C++ but the C api has no setter for it. without it I cant turn on vision input from Obj-C / Swift, so the model just ignores images even with a multimodal .litertlm loaded.

added litert_lm_engine_settings_set_max_num_images(settings, n) that just forwards to the C++ setter. default stays 0 (vision off), set to >0 to enable.

let me know if you want a different name.

thanks

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.