Use trimmed headers instead of cocoapods for Apple targets

[buenaflor]

I decided to keep using cocoapods because of several reasons:

• When I started 'rebuilding' a couple classes in custom objc headers such as SentryEvent I noticed how many classes we actually need and at some point we would maybe shave off 4-5 files (rough guess, cant remember exactly now) if we did it all by hand, which is not a lot.
• The total size of the klib file which contains the bindings that were created by the cocoapods plugin is around 70kb
• Future maintenance just sounds awful when we have to manually add/remove stuff when things are deprecated or added and it's easy to make a mistake thus crashing the users app
• Modularized sentry-cocoa may come soon-ish

What we should do: combine all the cinterop libraries that we had into one SentryInternal and shave off a lot of objc headers that aren't needed