ref(tracing): Move getBaggage() from Span to Transaction class

[Lms24]

As already discussed in #5292, this PR moves the getBaggage() method from the Span class, to the Transaction class. This change is good for two reasons:

1. It conceptually makes more sense to store DSC on the transaction rather than the span because we're always talking about transactions in DS
2. We can get rid of a sketchy type cast and avoid a circular dependency situation (transaction -> span -> transaction) entirely

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.39 KB (added)
@sentry/browser - ES5 CDN Bundle (minified)	59.95 KB (added)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.18 KB (added)
@sentry/browser - ES6 CDN Bundle (minified)	53.57 KB (added)
@sentry/browser - Webpack (gzipped + minified)	19.96 KB (added)
@sentry/browser - Webpack (minified)	65.02 KB (added)
@sentry/react - Webpack (gzipped + minified)	19.98 KB (added)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.22 KB (added)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.79 KB (added)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.25 KB (added)