ref(integrations): Refactor pluggable integrations to avoid setupOnce

[mydea]

Slowly getting rid of getCurrentHub()...

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	75.35 KB (+0.28% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	66.76 KB (+0.29% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	60.35 KB (+0.31% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.42 KB (+0.63% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	30.02 KB (+0.59% 🔺)
@sentry/browser - Webpack (gzipped)	21.72 KB (+0.89% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	72.74 KB (+0.24% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	64.46 KB (+0.29% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	30.74 KB (+0.64% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	22.78 KB (+0.88% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	202.58 KB (+0.21% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	92.68 KB (+0.45% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	67.72 KB (+0.62% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	33.58 KB (+0.56% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	67.13 KB (+0.31% 🔺)
@sentry/react - Webpack (gzipped)	21.76 KB (+0.88% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	83.91 KB (+0.25% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	48.58 KB (+0.41% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	16.35 KB (+1.06% 🔺)