feat(core): Add addIntegration method to client

[mydea]

This can be used to add an integration at runtime.

This is useful e.g. for replay, where we may want to lazy load this (as the payload is quite large).

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.84 KB (+0.11% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	61.43 KB (+0.15% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.61 KB (+0.09% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.98 KB (+0.13% 🔺)
@sentry/browser - Webpack (gzipped + minified)	20.38 KB (+0.11% 🔺)
@sentry/browser - Webpack (minified)	66.62 KB (+0.14% 🔺)
@sentry/react - Webpack (gzipped + minified)	20.41 KB (+0.1% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.65 KB (+0.05% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.81 KB (+0.11% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.25 KB (+0.11% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	42.99 KB (-0.02% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.24 KB (+0.01% 🔺)

[Lms24]

Nice, looks like bundle size impact is negligible so I'd say let's go for it!