Update available to fix build for non-macOS Apple platforms (#1746)

## Motivation

The package currently doesn't build for non-macOS Apple platforms, e.g. iOS, because of missing `@available` annotations, mostly in tests.

## Modifications

- Add missing `@available` annotations.
- Use `#if os(macOS) || os(Linux)` in test utils that require `Foundation.Process`. Ideally we'd use `#if canImport(Foundation.Process)`, but the version of `swift-format` used by this project doesn't understand it.

## Result

Code and tests can build for, and run on, other platforms, e.g. iOS.

Author: simonjbeaumont

Sources/Examples/PacketCapture/PacketCapture.swift

@@ -21,7 +21,7 @@ import NIOExtras
 import NIOPosix
 
 @main
-@available(macOS 10.15, *)
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 struct PCAP: AsyncParsableCommand {
   @Option(help: "The port to connect to")
   var port = 1234

Tests/GRPCCodeGenTests/Internal/Translator/SnippetBasedTranslatorTests.swift

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#if os(macOS) || os(Linux)  // swift-format doesn't like canImport(Foundation.Process)
+
 import XCTest
 
 @testable import GRPCCodeGen
@@ -517,3 +519,5 @@ extension SnippetBasedTranslatorTests {
     )
   }
 }
+
+#endif  // os(macOS) || os(Linux)

Tests/GRPCCodeGenTests/Internal/Translator/TestFunctions.swift

@@ -26,6 +26,9 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+
+#if os(macOS) || os(Linux)  // swift-format doesn't like canImport(Foundation.Process)
+
 import XCTest
 
 private func diff(expected: String, actual: String) throws -> String {
@@ -65,3 +68,5 @@ internal func XCTAssertEqualWithDiff(
     line: line
   )
 }
+
+#endif  // os(macOS) || os(Linux)

Tests/GRPCCoreTests/Call/Client/ClientResponseTests.swift

@@ -18,6 +18,7 @@ import XCTest
 
 @testable import GRPCCore
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 final class ClientResponseTests: XCTestCase {
   func testAcceptedSingleResponseConvenienceMethods() {
     let response = ClientResponse.Single(

Tests/GRPCCoreTests/Call/Server/ServerRequestTests.swift

@@ -16,6 +16,7 @@
 @_spi(Testing) import GRPCCore
 import XCTest
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 final class ServerRequestTests: XCTestCase {
   func testSingleToStreamConversion() async throws {
     let single = ServerRequest.Single(metadata: ["bar": "baz"], message: "foo")

Tests/GRPCCoreTests/Call/Server/ServerResponseTests.swift

@@ -16,6 +16,7 @@
 @_spi(Testing) import GRPCCore
 import XCTest
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 final class ServerResponseTests: XCTestCase {
   func testSingleConvenienceInit() {
     var response = ServerResponse.Single(

Tests/GRPCCoreTests/ServerErrorTests.swift

@@ -16,6 +16,7 @@
 import GRPCCore
 import XCTest
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 final class ServerErrorTests: XCTestCase {
   func testCopyOnWrite() {
     // ServerError has a heap based storage, so check CoW semantics are correctly implemented.

Tests/GRPCCoreTests/Streaming/Internal/AsyncSequenceOfOne.swift

@@ -18,6 +18,7 @@ import XCTest
 
 @testable import GRPCCore
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 internal final class AsyncSequenceOfOneTests: XCTestCase {
   func testSuccessPath() async throws {
     let sequence = RPCAsyncSequence.one("foo")

Tests/GRPCCoreTests/Streaming/Internal/BufferedStreamTests.swift

@@ -1080,6 +1080,7 @@ final class BufferedStreamTests: XCTestCase {
   }
 }
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension BufferedStream.Source.WriteResult {
   func assertIsProducerMore() {
     switch self {

Tests/GRPCCoreTests/Test Utilities/AsyncSequence+Utilities.swift

@@ -14,13 +14,15 @@
  * limitations under the License.
  */
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension AsyncSequence {
   func collect() async throws -> [Element] {
     return try await self.reduce(into: []) { $0.append($1) }
   }
 }
 
 #if swift(<5.9)
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension AsyncStream {
   static func makeStream(
     of elementType: Element.Type = Element.self,
@@ -34,6 +36,7 @@ extension AsyncStream {
   }
 }
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension AsyncThrowingStream {
   static func makeStream(
     of elementType: Element.Type = Element.self,