feat(core): Add optional setup hook to integrations

[mydea]

This can be used to replace some of our setupOnce hooks, and does not rely on global state.

I think with this, we are mostly done with the minimal integration API changes for v8. After this, we can:

1. update setupOnce to not get any arguments (you can just import and use getCurrentHub or whatever if you need it) - this we'll probably do only in v8 itself
2. Add a new functional form for the integrations. We can support both in v7 and then drop the class form in v8. So integrations would look like this:

export class Console implements Integration {
  public static id = 'Console';
  public name Console.id;
  public constructor(options) {}
  public setupOnce() {}
}

becomes

export const consoleIntegration: IntegrationFn = (options) => {
  return {
    setupOnce() {}
  }
}

And usage becomes:

Sentry.init({
  integrations: [
    // old
    new Sentry.Integrations.Console(options),
    // new
    Sentry.consoleIntegration(options)
   ]
})

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	65.43 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	55.64 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	30.99 KB (+0.04% 🔺)
@sentry/browser - Webpack (gzipped)	21.31 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	61.96 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.11 KB (+0.04% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.25 KB (+0.07% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	195.46 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	88.37 KB (+0.06% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	63.34 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.82 KB (+0.04% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.78 KB (+0.02% 🔺)
@sentry/react - Webpack (gzipped)	21.36 KB (+0.06% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	82.5 KB (+0.02% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	48.14 KB (+0.03% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	16 KB (0%)

[AbhiPrasad]

Nice good change!