Add top level Package.swift file (#24)

## What's Changed

Adding top level Package.swift file. Arrow and ArrowFlight are included
in the top level build.

Closes #15.

Author: abandy

Package.swift

@@ -0,0 +1,92 @@
+// swift-tools-version: 5.10
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you 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 PackageDescription
+
+let package = Package(
+    name: "ArrowSwift",
+    platforms: [
+        .macOS(.v10_15)
+    ],
+    products: [
+        .library(
+            name: "Arrow",
+            targets: ["Arrow"])
+    ],
+    dependencies: [
+        .package(url: "https://github.com/google/flatbuffers.git", branch: "v25.2.10"),
+        .package(url: "https://github.com/grpc/grpc-swift.git", from: "1.25.0"),
+        .package(url: "https://github.com/apple/swift-protobuf.git", from: "1.29.0"),
+        .package(
+            url: "https://github.com/apple/swift-atomics.git",
+            .upToNextMajor(from: "1.2.0") // or `.upToNextMinor
+        )
+    ],
+    targets: [
+        .target(
+            name: "ArrowC",
+            path: "Arrow/Sources/ArrowC",
+            swiftSettings: [
+                // build: .unsafeFlags(["-warnings-as-errors"])
+            ]
+
+        ),
+        .target(
+            name: "Arrow",
+            dependencies: ["ArrowC",
+                           .product(name: "FlatBuffers", package: "flatbuffers"),
+                           .product(name: "Atomics", package: "swift-atomics")
+            ],
+            path: "Arrow/Sources/Arrow",
+            swiftSettings: [
+                // build: .unsafeFlags(["-warnings-as-errors"])
+            ]
+        ),
+        .target(
+            name: "ArrowFlight",
+            dependencies: [
+                "Arrow",
+                .product(name: "GRPC", package: "grpc-swift"),
+                .product(name: "SwiftProtobuf", package: "swift-protobuf")
+            ],
+            path: "ArrowFlight/Sources/ArrowFlight",
+            swiftSettings: [
+                // build: .unsafeFlags(["-warnings-as-errors"])
+            ]
+        ),
+        .testTarget(
+            name: "ArrowTests",
+            dependencies: ["Arrow", "ArrowC"],
+            path: "Arrow/Tests",
+            swiftSettings: [
+                // build: .unsafeFlags(["-warnings-as-errors"])
+            ]
+        ),
+        .testTarget(
+            name: "ArrowFlightTests",
+            dependencies: ["ArrowFlight"],
+            path: "ArrowFlight/Tests",
+            swiftSettings: [
+                // build: .unsafeFlags(["-warnings-as-errors"])
+            ]
+        )
+
+    ]
+)