title: Podspec debug/release `vendored_framework` variants
body: <p>I have a flutter plugin (let's say it's named <code>plugin</code>) that needs to have a separate debug build (say <code>debug/foo.framework</code>) and a release build (<code>release/foo.framework</code>) and they should be used in the respective app builds. Is there a way to do this?</p>
  <pre><code>Pod::Spec.new do |s|
    ...
    s.xcconfig = { 'OTHER_LDFLAGS' =&gt; '-framework foo' }
    s.vendored_frameworks = 'foo.framework'
  end
  </code></pre>
  <p>What I'd want, conceptually, is something like this (should work for <code>dependency</code>, but doesn't for <code>vendored_framework</code> - says <code>file: undefined method 'vendored_frameworks'</code>)</p>
  <pre><code>Pod::Spec.new do |s|
    ...
    s.xcconfig = { 'OTHER_LDFLAGS' =&gt; '-framework foo' }
    s.vendored_frameworks 'debug/foo.framework', :configurations =&gt; ['Debug']
    s.vendored_frameworks 'release/foo.framework', :configurations =&gt; ['Release']
  </code></pre>
  <p>I've also tried this, but it fails with circular dependency:</p>
  <pre><code>Pod::Spec.new do |s|
    s.name     = 'plugin'
    ...
    s.xcconfig = { 'OTHER_LDFLAGS' =&gt; '-framework foo' }
  
    s.subspec 'debug' do |cs|
      cs.vendored_frameworks = 'debug/foo.framework'
    end
    s.subspec 'release' do |cs|
      cs.vendored_frameworks = 'release/foo.framework'
    end
  
    s.default_subspecs = :none
    s.dependency 'plugin/debug', :configurations =&gt; ['Debug']
    s.dependency 'plugin/release', :configurations =&gt; ['Release']
  end
  </code></pre>