IAM lifecycle events

[emawby]

This PR adds the OSInAppMessageLifecycleHandler protocol with the following lifecycle methods:

- (void)onWillDisplayInAppMessage:(OSInAppMessage *)message;
- (void)onDidDisplayInAppMessage:(OSInAppMessage *)message;
- (void)onWillDismissInAppMessage:(OSInAppMessage *)message;
- (void)onDidDismissInAppMessage:(OSInAppMessage *)message;

And adds the public setter method setInAppMessageLifecycleHandler

These methods should each be called once per display of an In App Message.

The InAppMessageDelegate was exposed in the public header with the method
- (void)handleMessageAction:(OSInAppMessageAction * _Nonnull)action
But it appears to never be called. I am leaving it in case it is erroneously being used by a consumer application.

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This change is 

[jkasten2]

Reviewed 28 of 28 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @emawby and @nan-li)

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iOS_SDK/OneSignalSDK/UnitTests/InAppMessagingIntegrationTests.m, line 1599 at r1 (raw file):

    let firstTrigger = [OSTrigger customTriggerWithProperty:@"testProp" withOperator:OSTriggerOperatorTypeExists withValue:nil];

    OSInAppMessageInternal * message = [OSInAppMessageTestHelper testMessageWithTriggers:@[@[firstTrigger]]];

nit, extra space around *.