Make Nexus work out of the box with zero config

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

https://github.com/temporalio/temporal/blob/16b1e29b2a86eaf1a95e026e49e16f31f8c414aa/components/callbacks/fx.go#L45
https://github.com/temporalio/temporal/blob/16b1e29b2a86eaf1a95e026e49e16f31f8c414aa/components/nexusoperations/fx.go#L102-L111