ref(tracing): Add necessary helpers for using propagation context on outgoing headers

[AbhiPrasad]

experiments here: #8433

In preparation for Part 3 of #8352 (which involves creating sentry-trace/baggage when a span does not exist and attaching them to outgoing requests/meta tags), this PR adds some helpers. Notably it adds:

1. A getter for PropagationContext on the scope
2. generateSentryTraceHeader, which will be used to dynamically create sentry-trace headers, regardless of if there is a span or not
3. getDynamicSamplingContextFromClient, which is used to generate dynamic sampling context from a client directly (instead of having to go through a transaction)

This PR also updates extractTraceparentData and dynamicSamplingContextToSentryBaggageHeader to be more liberal about the values it takes so we can better accommodate the new flows we are going to add.

Next up - we're updating Undici/Node HTTP! Then we move on to Next.js, Remix, Sveltekit, Serverless - and we should be good to go 🚀

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.79 KB (+0.08% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	68.55 KB (+0.16% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	20.1 KB (+0.1% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	59.79 KB (+0.14% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.71 KB (+0.12% 🔺)
@sentry/browser - Webpack (minified)	70.93 KB (+0.14% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.73 KB (+0.07% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	50.22 KB (-0.09% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	29.76 KB (-0.02% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	27.82 KB (-0.07% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	49.44 KB (+0.02% 🔺)
@sentry/replay - Webpack (gzipped + minified)	43.12 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	69.2 KB (-0.03% 🔽)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	61.66 KB (+0.03% 🔺)