ref(tracing): Unify DSC key names in envelope and baggage headers

[Lms24]

This PR unifies the key names in the envelope trace header and the baggage http header. As a result, we get rid of the mapping that was necessary before, thereby loosing a little JS bundle weight ;)
Furthermore, the PR renames the EventTraceContext interface and aligns the Baggage type definition so that we can ensure that both DSC transport mechanisms are based on the same keys.

Since EventTraceContext was exported before and therefore publicly accesible, I left the export as an alias of DynamicSamplingContext. If this is approved, I'll add that export to (#5194).
(Happy to discuss and adjust any of the changes)

fixes #5301

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.3 KB (added)
@sentry/browser - ES5 CDN Bundle (minified)	59.73 KB (added)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.11 KB (added)
@sentry/browser - ES6 CDN Bundle (minified)	53.37 KB (added)
@sentry/browser - Webpack (gzipped + minified)	19.89 KB (added)
@sentry/browser - Webpack (minified)	64.81 KB (added)
@sentry/react - Webpack (gzipped + minified)	19.92 KB (added)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.07 KB (added)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.66 KB (added)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.12 KB (added)

[AbhiPrasad]

Way cleaner!