[native] Add function to open native picker

Summary:
Created a function that opens the native image picker interface using the expo-image-picker library and then processes the result to be compatible with our MediaLibrarySelection type. Finally, it updates the component state to make the selection prepended to the MediaGalleryKeyboard and queued to send.

Useful docs: image picker options, returned data.

Depends on D6493, D6494

Test Plan:
Created a test button that triggers this function and ensured (both platforms) that:

• native picker opens
• multiple results can be picked
• selected media are prepended to the gallery and enqueued to send

Reviewers: ashoat, atul, inka, kamil

Reviewed By: ashoat, atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6495