Allow `getPodSpecs` to support the new "plugin" config element

# Feature Request

This is related to https://github.com/apache/cordova-ios/issues/1256

For that feature to be completed in the `cordova-ios` project, the `cordova-common` project must support the additional attribute in the podspec section of the plugin.xml

Start by locating getPodSpecs and the podspec section handling in plugin.xml. Read the related cordova-ios issue 1256 to understand the new "plugin" config element and how cordova-common is expected to represent it. Done means getPodSpecs supports that additional attribute for the cordova-ios feature.

Written by the indexing model from the issue text.