feat: Expose required things for React Native auto tracing

[jennmueng]

Exposes

registerRequestInstrumentation,
RequestInstrumentationOptions,
defaultRequestInstrumentationOptions,
IdleTransaction,
startIdleTransaction

for React Native, see this PR: getsentry/sentry-react-native#1230

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	19.97 KB (0%)
@sentry/browser - Webpack	20.8 KB (0%)
@sentry/react - Webpack	20.8 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.12 KB (+0.01% 🔺)

[giautm]

Why a simple pull request take too long time to review - two weeks? 👎 No one else in office?