Add Action and WorkflowRule definitions (#554)

<!-- Describe what has changed in this PR -->
**What changed?**
* Add Action proto definition
* Add WorkflowRuleSpec/WorkflowRule proto definitions

<!-- Tell your future self why have you made these changes -->
**Why?**
Part of WorkflowRules workstream.

Changes are reviewed in [this
PR](#554)

temporal/api/enums/v1/common.proto

@@ -104,3 +104,12 @@ enum NexusOperationCancellationState {
     // Cancellation request is blocked (eg: by circuit breaker).
     NEXUS_OPERATION_CANCELLATION_STATE_BLOCKED = 6;
 }
+
+enum RuleActionScope {
+    // Default value, unspecified scope.
+    RULE_ACTION_SCOPE_UNSPECIFIED = 0;
+    // The action will be applied to the entire workflow.
+    RULE_ACTION_SCOPE_WORKFLOW = 1;
+    // The action will be applied to a specific activity.
+    RULE_ACTION_SCOPE_ACTIVITY = 2;
+}

temporal/api/rules/v1/message.proto

@@ -0,0 +1,99 @@
+// The MIT License
+//
+// Copyright (c) 2025 Temporal Technologies Inc.  All rights reserved.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+syntax = "proto3";
+
+package temporal.api.rules.v1;
+
+option go_package = "go.temporal.io/api/rules/v1;rules";
+option java_package = "io.temporal.api.rules.v1";
+option java_multiple_files = true;
+option java_outer_classname = "MessageProto";
+option ruby_package = "Temporalio::Api::Rules::V1";
+option csharp_namespace = "Temporalio.Api.Rules.V1";
+
+
+import "google/protobuf/timestamp.proto";
+import "temporal/api/enums/v1/common.proto";
+
+message Action {
+  // Pause action.
+  message ActionPause {
+    // Indicates what should be paused. Depending on the scope value, the action
+    // will pause the entire workflow or a specific activity (additional entities
+    // may be supported in the future).
+    temporal.api.enums.v1.RuleActionScope scope = 1;
+  }
+
+  // Supported actions.
+  oneof variant  {
+    ActionPause pause = 1;
+  }
+}
+
+message WorkflowRuleSpec {
+  // The id of the new workflow rule. Must be unique within the namespace.
+  string id = 1;
+
+  // Activity trigger will be triggered when an activity is about to start.
+  message ActivityStartTrigger {
+    // Activity predicate is used to match against workflow data, based on rule trigger type.
+    // The following activity attributes are supported as part of the predicate:
+    // - ActivityType: The type of the activity.
+    // - ActivityId: The ID of the activity.
+    // - ActivityAttempt: The number attempts of the activity.
+    // - BackoffInterval: The current backoff interval of the activity.
+    // - ActivityStatus: The status of the activity. Can be one of "scheduled", "started", "paused".
+    // - TaskQueue: The name of the task queue of the activity.
+    string predicate = 1;
+  }
+
+  // Specifies how the rule should be triggered and evaluated.
+  // Currently, only "activity start" type is supported.
+  oneof trigger {
+    ActivityStartTrigger activity_start = 2;
+  }
+
+  // Restricted Visibility query.
+  // The following activity attributes are supported as part of the predicate:
+  // - WorkflowType
+  // - WorkflowId
+  // - StartTime
+  // - ExecutionStatus
+  string visibility_query = 3;
+
+  // Action to be taken when the rule is triggered and predicate is matched.
+  repeated Action actions = 4;
+
+  // Expiration time of the rule. After this time, the rule will be deleted.
+  // Can be empty if the rule should never expire.
+  google.protobuf.Timestamp expiration_time = 5;
+}
+
+// WorkflowRule describes a rule that can be applied to any workflow in this namespace.
+message WorkflowRule {
+  // Rule creation date.
+  google.protobuf.Timestamp create_time = 1;
+
+  // Rule specification
+  WorkflowRuleSpec spec = 2;
+}