ref: Remove forward declarations for Envelope (#3732)

Having unused forward declarations in the SentryEnvelope.h doesn't make
sense and makes it harder to expose that specific class to Swift.
Therefore, we can remove them.

Author: philipphofmann

Sources/Sentry/include/HybridPublic/SentryEnvelope.h

@@ -8,8 +8,12 @@
 
 #endif
 
-@class SentryEvent, SentrySession, SentryId, SentryUserFeedback, SentryAttachment,
-    SentryTransaction, SentryClientReport, SentryEnvelopeItemHeader;
+@class SentryEvent;
+@class SentrySession;
+@class SentryId;
+@class SentryUserFeedback;
+@class SentryAttachment;
+@class SentryEnvelopeItemHeader;
 
 NS_ASSUME_NONNULL_BEGIN
 

Sources/Sentry/include/SentryEnvelope+Private.h

@@ -2,6 +2,8 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+@class SentryClientReport;
+
 @interface
 SentryEnvelopeItem ()