feat(caddystorage): define CaddyStorage gRPC service API for distributed Caddy storage

internal/machine/api/pb/caddy_storage.proto

@@ -0,0 +1,74 @@
+syntax = "proto3";
+
+package api;
+
+option go_package = "github.com/psviderski/uncloud/internal/machine/api/pb";
+
+import "google/protobuf/empty.proto";
+import "google/protobuf/timestamp.proto";
+import "internal/machine/api/pb/common.proto";
+
+// CaddyStorage exposes the CertMagic storage operations backed by the distributed cluster store.
+service CaddyStorage {
+  rpc Store(StoreCaddyStorageRequest) returns (google.protobuf.Empty);
+  rpc Load(LoadCaddyStorageRequest) returns (LoadCaddyStorageResponse);
+  rpc Delete(DeleteCaddyStorageRequest) returns (EmptyResponse);
+  rpc List(ListCaddyStorageRequest) returns (ListCaddyStorageResponse);
+  rpc Stat(StatCaddyStorageRequest) returns (StatCaddyStorageResponse);
+}
+
+message StoreCaddyStorageRequest {
+  string key = 1;
+  bytes value = 2;
+}
+
+message LoadCaddyStorageRequest {
+  string key = 1;
+}
+
+message LoadCaddyStorageResponse {
+  // Must contain only one repeated messages field to allow broadcasting Load requests to multiple machines.
+  repeated MachineCaddyStorageValue messages = 1;
+}
+
+message MachineCaddyStorageValue {
+  Metadata metadata = 1;
+  bytes value = 2;
+  google.protobuf.Timestamp updated_at = 3;
+}
+
+message DeleteCaddyStorageRequest {
+  string key = 1;
+}
+
+message ListCaddyStorageRequest {
+  string prefix = 1;
+  bool recursive = 2;
+}
+
+message ListCaddyStorageResponse {
+  // Must contain only one repeated messages field to allow broadcasting List requests to multiple machines.
+  repeated MachineCaddyStorageKeys messages = 1;
+}
+
+message MachineCaddyStorageKeys {
+  Metadata metadata = 1;
+  repeated string keys = 2;
+}
+
+message StatCaddyStorageRequest {
+  string key = 1;
+}
+
+message StatCaddyStorageResponse {
+  // Must contain only one repeated messages field to allow broadcasting Stat requests to multiple machines.
+  repeated MachineCaddyStorageKeyInfo messages = 1;
+}
+
+message MachineCaddyStorageKeyInfo {
+  Metadata metadata = 1;
+  string key = 2;
+  google.protobuf.Timestamp updated_at = 3;
+  int64 size = 4;
+  bool is_terminal = 5;
+}