API for adding triggers has string values

* addTrigger and addTriggers public API will only accept string values, not types string/bool/number, etc.
* No logic changes are needed as the SDK already supports string-only trigger values and evaluates booleans and numerics correctly.

Tested scenarios with these triggers set via the dashboard on IAMs:
🟢 myTrigger is true
🟡 myTrigger is 1.58 (must pass in "1.58" and not "1.580" for example, the "is" operator does not support numeric equivalence, it arrives to the SDK as value "1.58")
🟢 myTrigger > 5
🟢 myTrigger > 7.689
🟡 myTrigger >= 7.689 (note margin of error due to rounding, this value actually came to the SDK as 7.689000129699707, so adding a trigger value of "7.689" does not trigger the IAM to display)

Author: nan-li

iOS_SDK/OneSignalSDK/Source/OneSignalInAppMessaging.h

@@ -90,10 +90,13 @@
 - (void)onDidDismissInAppMessage:(OSInAppMessage *_Nonnull)message;
 @end
 
+/**
+ Public API for the InAppMessages namespace.
+ */
 @protocol OSInAppMessages <NSObject>
 
-+ (void)addTrigger:(NSString * _Nonnull)key withValue:(id _Nonnull)value;
-+ (void)addTriggers:(NSDictionary<NSString *, id> * _Nonnull)triggers;
++ (void)addTrigger:(NSString * _Nonnull)key withValue:(NSString * _Nonnull)value;
++ (void)addTriggers:(NSDictionary<NSString *, NSString *> * _Nonnull)triggers;
 + (void)removeTrigger:(NSString * _Nonnull)key;
 + (void)removeTriggers:(NSArray<NSString *> * _Nonnull)keys;
 + (void)clearTriggers;

iOS_SDK/OneSignalSDK/Source/OneSignalInAppMessaging.m

@@ -49,7 +49,7 @@ + (void)setLifecycleHandler:(NSObject<OSInAppMessageLifecycleHandler> *_Nullable
     [OSMessagingController.sharedInstance setInAppMessageDelegate:delegate];
 }
 
-+ (void)addTrigger:(NSString * _Nonnull)key withValue:(id _Nonnull)value {
++ (void)addTrigger:(NSString * _Nonnull)key withValue:(NSString * _Nonnull)value {
     // return if the user has not granted privacy permissions
     if ([OSPrivacyConsentController shouldLogMissingPrivacyConsentErrorWithMethodName:@"addTrigger:withValue:"])
         return;
@@ -62,7 +62,7 @@ + (void)addTrigger:(NSString * _Nonnull)key withValue:(id _Nonnull)value {
     [OSMessagingController.sharedInstance addTriggers:@{key : value}];
 }
 
-+ (void)addTriggers:(NSDictionary<NSString *,id> * _Nonnull)triggers {
++ (void)addTriggers:(NSDictionary<NSString *, NSString *> * _Nonnull)triggers {
     // return if the user has not granted privacy permissions
     if ([OSPrivacyConsentController shouldLogMissingPrivacyConsentErrorWithMethodName:@"addTriggers:"])
         return;