Support extending export options

[dertieran]

Feature Request

Motivation Behind Feature

When building the ios app I want to set some additional exportOptions, for example set uploadSymbols to false. (I need this to not include symbols in my build when uploading it to the app store)

Feature Description

Basically I would want something like this (Depending on if it is useful to overwrite defaults or not)

const exportOptions = { ...buildOpts.exportOptions, compileBitcode: false, method: 'development' };

Alternatives or Workarounds

Right now I need to run xcodebuild manually instead of using the build-in functionality.