feat(replays): add replay_id to dynamic sampling context

[JoshFerge]

Only used for testing // as a reference for what we want to do. need to determine how to do this while preserving tree shaking, etc.

[AbhiPrasad]

We should do this via hooks:

sentry-javascript/packages/core/src/baseclient.ts

Line 364 in af0224a

// Keep on() & emit() signatures in sync with types' client.ts interface

So:

// packages/core/src/tracing/transaction.ts
const client = getCurrentHub().getClient();
client.emit('dscCreate', dsc);

And then in Replay:

// packages/replay/src/integration.ts
const client = getCurrentHub().getClient();
client.on('dscCreate', (dsc) => {
  if (!this._replay || !this._replay.session) {
    return;
  }
  dsc.replay_id = this._replay.session.id;
});

This will mean Replay is treeshaken if not used, and will have minimal bundle size impact.