Allow using HockeyAppSDK pod subspecs

[ajmath]

For my use case, I need to use both HockeyApp and AppCenter pods (AppCenter doesn't support feedback yet). Unfortunately, including both causes an error at compile time due to overlapping symbols. The solution was to use pod 'HockeySDK', :subspecs => 'FeedbackOnlyLib' to only include the feedback pod spec. However, when doing this RNHockeyApp.m had multiple compilation errors due to referencing code that was not included via cocoapods.

This PR adds the appropriate #if HOCKEYSDK_FEATURE_... flags to allow compiling the react-native module with a subset of the HockeyAppSDK pod subspecs.