feat(browser): Add user agent to INP standalone span attributes

[Lms24]

This PR adds the window.navigator.userAgent string as the user_agent.original attribute to INP standalone spans.

I decided to specifically add this attribute in the INP handler when starting the span for now because we can generalize this to more standalone spans later on if we need it. Saves us a bit of bundle size for the moment.

This is part of making INP value collection more robust by ensuring we send the user agent also when tunneling is used where the user agent usually isn't forwarded to the Sentry backend.

closes #12689

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	22.3 KB (0%)
@sentry/browser (incl. Tracing)	33.69 KB (+0.06% 🔺)
@sentry/browser (incl. Tracing, Replay)	69.47 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	62.8 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas)	73.52 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback)	86.18 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback, metrics)	88.03 KB (+0.03% 🔺)
@sentry/browser (incl. metrics)	26.59 KB (0%)
@sentry/browser (incl. Feedback)	38.98 KB (0%)
@sentry/browser (incl. sendFeedback)	26.92 KB (0%)
@sentry/browser (incl. FeedbackAsync)	31.54 KB (0%)
@sentry/react	25.07 KB (0%)
@sentry/react (incl. Tracing)	36.75 KB (+0.05% 🔺)
@sentry/vue	26.41 KB (0%)
@sentry/vue (incl. Tracing)	35.57 KB (+0.06% 🔺)
@sentry/svelte	22.44 KB (0%)
CDN Bundle	23.52 KB (0%)
CDN Bundle (incl. Tracing)	35.47 KB (+0.07% 🔺)
CDN Bundle (incl. Tracing, Replay)	69.57 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	74.77 KB (+0.03% 🔺)
CDN Bundle - uncompressed	69 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	104.91 KB (+0.06% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	215.3 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	228.02 KB (+0.03% 🔺)
@sentry/nextjs (client)	36.62 KB (+0.05% 🔺)
@sentry/sveltekit (client)	34.34 KB (+0.05% 🔺)
@sentry/node	130.8 KB (0%)
@sentry/node - without tracing	91.88 KB (0%)
@sentry/aws-serverless	117.06 KB (0%)