Add `ServiceDescriptor` to GRPCCore (#1982)

Motivation:
`ServiceDescriptor` describes a service. It is needed for services that interact with the Health service.

Modifications:
- Add a `public struct ServiceDescriptor` to GRPCCore.

Result:
Services will be describable using `ServiceDescriptor`.

Sources/GRPCCore/ServiceDescriptor.swift

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/// A description of a service.
+public struct ServiceDescriptor: Sendable, Hashable {
+  /// The name of the package the service belongs to. For example, "helloworld".
+  /// An empty string means that the service does not belong to any package.
+  public var package: String
+
+  /// The name of the service. For example, "Greeter".
+  public var service: String
+
+  /// The fully qualified service name in the format:
+  /// - "package.service": if a package name is specified. For example, "helloworld.Greeter".
+  /// - "service": if a package name is not specified. For example, "Greeter".
+  public var fullyQualifiedService: String {
+    if self.package.isEmpty {
+      return self.service
+    }
+
+    return "\(self.package).\(self.service)"
+  }
+
+  /// - Parameters:
+  ///   - package: The name of the package the service belongs to. For example, "helloworld".
+  ///   An empty string means that the service does not belong to any package.
+  ///   - service: The name of the service. For example, "Greeter".
+  public init(package: String, service: String) {
+    self.package = package
+    self.service = service
+  }
+}

Tests/GRPCCoreTests/ServiceDescriptorTests.swift

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import GRPCCore
+import XCTest
+
+final class ServiceDescriptorTests: XCTestCase {
+  func testFullyQualifiedName() {
+    let descriptor = ServiceDescriptor(package: "foo.bar", service: "Baz")
+    XCTAssertEqual(descriptor.package, "foo.bar")
+    XCTAssertEqual(descriptor.service, "Baz")
+    XCTAssertEqual(descriptor.fullyQualifiedService, "foo.bar.Baz")
+  }
+}