feat(dns): allow setting an externally managed cluster domain (uc dns set)

* feat: add uc dns set

This allows manual setting of the cluster's domain name. I use this to
make the local cluster admin aware that this has been done.
(and then manually update the external dns)

```
% uc dns set ctrl.u.science.ru.nl
Set cluster domain: ctrl.u.science.ru.nl
Deploy the Caddy reverse proxy service ('uc caddy deploy') to enable internet access to your services via the reserved or your custom domain.

% uc dns show
ctrl.u.science.ru.nl

% uc dns set ctrl.u.science.ru.nl
Error: domain already reserved

% uc dns release
Released cluster domain: ctrl.u.science.ru.nl
```

See: #37

Signed-off-by: Miek Gieben <miek@miek.nl>

* docs

Signed-off-by: Miek Gieben <miek@miek.nl>

* check validatity

Signed-off-by: Miek Gieben <miek@miek.nl>

* No need to call out to update anything, as set doesnt use uncloud dns

Signed-off-by: Miek Gieben <miek@miek.nl>

* Ignore domain when there is no endpoint

Prevents this error, when you know what you're doing:

    Error: failed to update DNS records pointing to caddy service: create cluster domain records in Uncloud DNS: rpc error: code = Unknown desc = Post "/domains/ctrl.u.science.ru.nl/records": unsupported protocol scheme ""

Signed-off-by: Miek Gieben <miek@miek.nl>

* from main

Signed-off-by: Miek Gieben <miek@miek.nl>

* mise proto && make cli-docs

Signed-off-by: Miek Gieben <miek@miek.nl>

* Rebase and and generate protos again

Signed-off-by: Miek Gieben <miek@miek.nl>

* Rebase and fix

Signed-off-by: Miek Gieben <miek@miek.nl>

* feat(dns): allow setting an externally managed cluster domain

---------

Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: Pasha Sviderski <me@psviderski.name>

cmd/uc/caddy/deploy.go

@@ -202,6 +202,10 @@ func UpdateDomainRecords(ctx context.Context, clusterClient *client.Client, prog
 		}
 		return fmt.Errorf("get cluster domain: %w", err)
 	}
+	if !domain.Reserved {
+		fmt.Printf("Skipping DNS records update as cluster domain '%s' is managed externally.\n", domain.Name)
+		return nil
+	}
 
 	fmt.Println("Updating cluster domain records in Uncloud DNS to point to machines running caddy service...")
 	// TODO: split the method into two: one to get the records and one to update them to ask for update confirmation.
@@ -216,7 +220,7 @@ func UpdateDomainRecords(ctx context.Context, clusterClient *client.Client, prog
 		if errors.Is(err, client.ErrNoReachableMachines) {
 			fmt.Println()
 			fmt.Printf("DNS records for domain '%s' could not be updated as there are no internet-reachable "+
-				"machines running caddy containers.\n", domain)
+				"machines running caddy containers.\n", domain.Name)
 			fmt.Println()
 			fmt.Println("Possible solutions:")
 			fmt.Println("- Ensure your machines have public IP addresses")