fix(nextjs): Proxy public key with tunnelRoute option

[lforst]

Fixes #6539

Replay events need a sentry_key query parameter. The current implementation of tunnelRoute doesn't proxy that query parameter leading to a 401 response from Sentry.

This PR fixes this issue by also proxying the sentry_key parameter. It is now a bit easier for adblockers to fingerprint for these proxy requests with the additional query parameter but we have to deal with that.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.78 KB (-0.02% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.27 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.56 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.8 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.34 KB (0%)
@sentry/browser - Webpack (minified)	66.48 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.36 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.61 KB (+0.08% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.75 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.19 KB (-0.01% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	41.89 KB (+0.02% 🔺)
@sentry/replay - Webpack (gzipped + minified)	38.12 KB (+0.02% 🔺)

[lforst]

Fixed via #6568 instead