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/dns/root.go

@@ -7,16 +7,19 @@ import (
 func NewRootCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "dns",
-		Short: "Manage cluster domain in Uncloud DNS.",
-		Long: "Manage cluster domain in Uncloud DNS.\n" +
+		Short: "Manage the cluster domain.",
+		Long: "Manage the cluster domain.\n" +
 			"DNS commands allow you to reserve or release a unique 'xxxxxx.uncld.dev' domain for your " +
 			"cluster. When reserved, Caddy service deployments will automatically update DNS records to route " +
-			"traffic to the services in the cluster.",
+			"traffic to the services in the cluster.\n\n" +
+			"EXPERIMENTAL: Use 'uc dns set' to configure an externally managed domain instead. " +
+			"Uncloud does not manage DNS records for manually set domains.",
 	}
 	cmd.AddCommand(
 		NewReleaseCommand(),
 		NewReserveCommand(),
 		NewShowCommand(),
+		NewSetCommand(),
 	)
 	return cmd
 }