bumped version number and added changelog for 3.4.0

[aboedo]

3.4.0

• Added proxyKey, useful for kids category apps, so that they can set up a proxy to send requests through. Do not use this unless you've talked to RevenueCat support about it.
  Proxy key #258
• Added managementURL to purchaserInfo. This provides an easy way for apps to create Manage Subscription buttons that will correctly redirect users to the corresponding subscription management page on all platforms.
  Management url #259
• Extra fields sent to the post receipt endpoint: normal_duration, intro_duration and trial_duration. These will feed into the LTV model for more accurate LTV values.
  subscription period #256
• Fixed a bug where if the appUserID was not found in NSUserDefaults and createAlias was called, the SDK would create an alias to (null).
  fix: (null) aliases #255
• Added mParticle as an option for attribution.
  add mParticle attribution as an option #251
• Fixed build warnings for Mac Catalyst
  Prevent a warning when compiling for Mac Catalyst #247
• Simplified Podspec and minor cleanup
  Cleanup rc cross platform support & podspec #248

[vegaro]

Lots of good stuff going out with this one!!!