Make Nexus work out of the box with zero config

[bergundy]

Start with

• Allow temporal://system URLs always to be attached as callbacks.
• Dispatch temporal://system callback URLs internally, automatically (see link below).

Then

• Add a token here.

Then update the SPEC and Nexus SDKs

• In the Nexus SPEC.md, call out that the Nexus-Callback-Token is a standard header and that it is required as part of the StartOperation request.
• In the Nexus SDKs (start with Go), add a CallbackToken []byte (or string TBD) field in StartOpertionOptions (called details, context) in the other SDKs.

Once the server can handle these new URLs, switch to using them

• Behind a dynamic config, which can be deprecated in a few releases.
• Default the server's callback URL to temporal://system.
  • The callback URL template, must be provided when making "external" calls but "worker" endpoints should use the temporal://system callback URL.
• Figure out how to make this work in Cloud.

Reference

temporal/components/callbacks/fx.go

Line 45 in 16b1e29

return func(r *http.Request) (*http.Response, error) {

temporal/components/nexusoperations/fx.go

Lines 102 to 111 in 16b1e29

	case *persistencespb.NexusEndpointTarget_External_:
	url = variant.External.GetUrl()
	var err error
	httpClient, err = m.Get(clientProviderCacheKey{namespaceID, entry.Id, url})
	if err != nil {
	return nil, err
	}
	case *persistencespb.NexusEndpointTarget_Worker_:
	url = cl.BaseURL() + "/" + commonnexus.RouteDispatchNexusTaskByEndpoint.Path(entry.Id)
	httpClient = &cl.Client

[chaptersix]

PR: #8308 for first task.