ref: Refactor transport, request, and client report types

[lforst]

This PR cleans up a few types we intend to use for client reports:

• Create EventDropReason in favor of Outcome. The term Outcome is a bit overloaded and the docs use it to describe the structure of the objects we should send as client report. We change this to better match the docs.
• Introduce DataCategory type in favor of TransportCategory and SentryRequestType. Data categories are used in more places than the transports and we should reflect that. This is also to better match the docs.
• Remove unused SentryRequest type.

Bonus:

• Tiny cleanup in createSessionEnvelope so we don't need a type-cast :)

Ref: https://getsentry.atlassian.net/browse/WEB-775

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.12 KB (-10.04% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	56.97 KB (-11.83% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	16.9 KB (-10.4% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	51.21 KB (-11.67% 🔽)
@sentry/browser - Webpack (gzipped + minified)	18.81 KB (-19.07% 🔽)
@sentry/browser - Webpack (minified)	60.99 KB (-25.36% 🔽)
@sentry/react - Webpack (gzipped + minified)	18.83 KB (-19.11% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	42.51 KB (-11.54% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	23.89 KB (-8.38% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	22.42 KB (-8.43% 🔽)

[Lms24]

Looking good, I like the types changes