feat(scope): Add client to scope

[AbhiPrasad]

Ref: #3751

As we move toward introducing a scope manager, we want to remove the idea of a stack of scopes and clients. As a replacement, the client now goes onto the scope. This enables patterns like:

Sentry.withScope((scope) => {
  scope.bindClient(TeamAClient);
  // any operations will use team A's client
});

By propagating the client through the scope, it becomes easier to devs to leverage multiple clients on the same page or process. This does mean that to capture events and messages, the hub will have to reach into the scope to access the client.

Temporarily, we store the client reference in both the hub stack and in the scope. This is to have backwards compatibility while we talk through the patterns and edit tests.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.42 KB (+0.13% 🔺)
@sentry/browser - Webpack	22.44 KB (+0.15% 🔺)
@sentry/react - Webpack	22.47 KB (+0.15% 🔺)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.91 KB (+0.1% 🔺)

[rhcarvalho]

@AbhiPrasad let's sync once more (on Monday) before we merge this.
As with #3753, I have a hunch we could limit the risk and number of changes by concentrating on better hub propagation (and leave the scope-centered idea to a major release).

[AbhiPrasad]

Closing as we have decided not to pursue this for now.