ref: Use SentrySpan protocol instead of class

[github-actions]

Note

The pull request "ref: Use SentrySpan protocol instead of class" was created by @noahsmartin but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

The SentrySpan protocol is already part of the public interface and can therefore be used from Swift. The SentrySpan class is still internal to objc. To make this Swift migration easier I changed it to use the protocol, which seems to compile fine

#skip-changelog

[linear]

COCOA-736 ref: Use SentrySpan protocol instead of class