Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

I have a project with static lib target, and it has no INFO_PLIST in build settings. And get_version_number fails to get MARKETING_VERSION from build settings

Description

Despite missing INFO_PLISToption, there still is a MARKETING_VERSION 😅.
So I added an option to skip looking for the value, or used variable in the Info.plist file if there is none. As the default value for looking in the build settings, I use MARKETING_VERSION, which is default from Xcode 11.

Testing Steps

I've updated specs for get_version_number to cover this case.

Manual steps:

1. Create target w/o INFO_PLIST build setting (static lib, aggregate).
2. Set MARKETING_VERSION build setting.
3. Run get_version_number on that target.