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>

api/pb/cluster.proto

@@ -14,6 +14,7 @@ service Cluster {
   rpc RemoveMachine(RemoveMachineRequest) returns (google.protobuf.Empty);
 
   rpc ReserveDomain(ReserveDomainRequest) returns (Domain);
+  rpc SetDomain(SetDomainRequest) returns (google.protobuf.Empty);
   rpc GetDomain(google.protobuf.Empty) returns (Domain);
   rpc ReleaseDomain(google.protobuf.Empty) returns (Domain);
   rpc CreateDomainRecords(CreateDomainRecordsRequest) returns (CreateDomainRecordsResponse);
@@ -56,12 +57,20 @@ message RemoveMachineRequest {
 
 message Domain {
   string name = 1;
+  // Whether the domain is reserved in Uncloud DNS. Older daemons (<0.21) omit this field
+  // and only support reserved domains, so clients should treat absence as true.
+  optional bool reserved = 2;
 }
 
 message ReserveDomainRequest {
   string endpoint = 1;
 }
 
+message SetDomainRequest {
+  // An externally managed domain name. Empty clears a manually set domain.
+  string name = 1;
+}
+
 message CreateDomainRecordsRequest {
   repeated DNSRecord records = 1;
 }