feat(core): Allow to pass forceTransaction to startSpan() APIs (backport)

[mydea]

This will ensure a span is sent as a transaction to Sentry.

This only implements this option for the core implementation, not yet for OTEL - that is a follow up here:
#10807

This is a backport to v7 of #10749

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	78.54 KB (+0.14% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.76 KB (+0.15% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	73.7 KB (+0.14% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	63.38 KB (+0.17% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.74 KB (+0.34% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.64 KB (+0.32% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.38 KB (0%)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.39 KB (0%)
@sentry/browser - Webpack (gzipped)	22.66 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.78 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	68.28 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	34.08 KB (+0.16% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	25.07 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	215.17 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	102.96 KB (+0.18% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	75.27 KB (+0.26% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	37.25 KB (+0.17% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	70.14 KB (+0.16% 🔺)
@sentry/react - Webpack (gzipped)	22.69 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	87.86 KB (+0.12% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	52.05 KB (+0.2% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.24 KB (0%)