api: add Token field to ServiceRegisterOpts (#18983)

Ongoing work to support Nomad Workload Identity for authenticating with Consul
will mean that Nomad's service registration sync with Consul will want to use
Consul tokens scoped to individual workloads for registering services and
checks. The `ServiceRegisterOpts` type in the API doesn't have an option to pass
the token in, which prevent us from sharing the same Consul connection for all
workloads. Add a `Token` field to match the behavior of `ServiceDeregisterOpts`.

.changelog/18983.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+api: added `Token` field to `ServiceRegisterOpts` type in Agent API
+```

api/agent.go

@@ -307,6 +307,10 @@ type ServiceRegisterOpts struct {
 	// having to manually deregister checks.
 	ReplaceExistingChecks bool
 
+	// Token is used to provide a per-request ACL token
+	// which overrides the agent's default token.
+	Token string
+
 	// ctx is an optional context pass through to the underlying HTTP
 	// request layer. Use WithContext() to set the context.
 	ctx context.Context
@@ -835,6 +839,9 @@ func (a *Agent) serviceRegister(service *AgentServiceRegistration, opts ServiceR
 	if opts.ReplaceExistingChecks {
 		r.params.Set("replace-existing-checks", "true")
 	}
+	if opts.Token != "" {
+		r.header.Set("X-Consul-Token", opts.Token)
+	}
 	_, resp, err := a.c.doRequest(r)
 	if err != nil {
 		return err

api/agent_test.go

@@ -297,6 +297,21 @@ func TestAgent_ServiceRegisterOpts_WithContextTimeout(t *testing.T) {
 	require.True(t, errors.Is(err, context.DeadlineExceeded), "expected timeout")
 }
 
+func TestAgent_ServiceRegisterOpts_Token(t *testing.T) {
+	c, s := makeACLClient(t)
+	defer s.Stop()
+
+	reg := &AgentServiceRegistration{Name: "example"}
+	opts := &ServiceRegisterOpts{}
+	opts.Token = "invalid"
+	err := c.Agent().ServiceRegisterOpts(reg, *opts)
+	require.EqualError(t, err, "Unexpected response code: 403 (ACL not found)")
+
+	opts.Token = "root"
+	err = c.Agent().ServiceRegisterOpts(reg, *opts)
+	require.NoError(t, err)
+}
+
 func TestAPI_NewClient_TokenFileCLIFirstPriority(t *testing.T) {
 	os.Setenv("CONSUL_HTTP_TOKEN_FILE", "httpTokenFile.txt")
 	os.Setenv("CONSUL_HTTP_TOKEN", "httpToken")