[iOS] add saveToGallery param on iOS for captureVideo

[chriskhongqarma]

iOS

Motivation and Context

Currently, the plugin saves the video file into gallery by default, which is not an expected behaviour in many cases. Developers should have the control over whether the file should be saved into gallery or not. The issue for this is described in #216

Description

A new param saveToGallery is added. We get the input configuration in captureVideo, and set it to a global variable. It will be used in processVideo method to decide whether the file should be saved into gallery or not.

Testing

set saveToGallery param to 0 and 1 when capturing video, seeing whether the file is saved into gallery. Make sure it works as intended.

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [x] I added automated test coverage as appropriate for this change
• [x] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [x] I've updated the documentation if necessary