Demonstration of how #14470 could be used in practice.

This PR allows for the runtime reconfiguration of the notification webhook endpoint.
In reality, what we'd actually want it for this value to be configurable on a per-organization level, but the plumbing to get the org details into the Dispatch function complicates the demonstration.

NOTE: in codersdk/deployment.go you'll notice at the bottom that we're iterating over all fields and calling Initialize(opt.Env):

// Initialize runtime fields.
for _, opt := range opts {
	d, ok := opt.Value.(runtimeconfig.Initializer)
	if !ok {
		continue
	}

	if opt.Env == "" {
		panic(fmt.Sprintf("developer error: option %q must have Env value defined to be runtime-configurable", opt.Name))
	}

	d.Initialize(opt.Env)
}

This is because we need to assign a name to each wrapped field. This name is used to identify the field in the database, and this is the simplest approach @Emyrk and I could come up with. We need some stable identity for each field, and the field's Env value is stable.

NOTE: I've protected func (DeploymentValues) Options() with a sync.OnceFunc because this method gets called in several places and it's both inefficient and dangerous to run this code several times; once is enough.

TestRuntimeEndpointChange demonstrates how it all comes together. Besides the changes in coderd/notifications/dispatch/webhook.go, this is mostly boilerplate.