feat(types): Add PolymorphicRequest type

[lobsterkatie]

This is a change which comes out of a code review of #5703, moving the CrossPlatformType from @sentry/utils to @sentry/types and renaming it PolymorphicRequest (to match the existing PolymorphicEvent). Because it affects a number of files, I decided to pull it into its own PR, just to not confuse things in that one.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.53 KB (+0.02% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.31 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.12 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	53.23 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.91 KB (0%)
@sentry/browser - Webpack (minified)	64.61 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.93 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.97 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.1 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.48 KB (+0.01% 🔺)

[AbhiPrasad]

Nice cleanup!

[vladanpaunovic]

Thanks for making this more explicit.

Please fix tests and linting. Then it's good to go.

[lobsterkatie]

Please fix tests and linting.

Yeah, of course. This is what I get for pushing right before I go to bed and not waiting for CI... I don't get a chance to fix all my dumb mistakes, LOL.