feat(PluginInfo): add podspec plugins support (#177)

[yinzara]

Platforms affected

iOS platform

Motivation and Context

This PR adds support for a new "plugins" and "plugin" elements in the "podspec" section of the plugin.xml to support the Cocoapods Plugins syntax standards:

Closes #177

Description

See description in https://github.com/apache/cordova-ios/issues/1256

Testing

Automated tests have been updated to reflect the change

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