ARROW-4435: Minor fixups to csharp .sln and .csproj file

1. Add .sln file to the repo.
2. Minor clean up of .csproj to move properties out of the Configuration/Platform PropertyGroups, which is the standard way of setting properties in SDK style projects.

Author: Eric Erhardt <eric.erhardt@microsoft.com>

Closes #3529 from eerhardt/SolutionProjectFixUp and squashes the following commits:

88a7c6e <Eric Erhardt> Add RAT exclude for csharp sln file
e9036b3 <Eric Erhardt> Minor fixups to csharp .sln and .csproj file

csharp/.gitignore

@@ -261,4 +261,7 @@ __pycache__/
 *.pyc
 
 # Project-specific
-artifacts/
+artifacts/
+
+# add .sln files back because they are ignored by the root .gitignore file
+!*.sln

csharp/Apache.Arrow.sln

@@ -0,0 +1,31 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio 15
+VisualStudioVersion = 15.0.28307.357
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Apache.Arrow", "src\Apache.Arrow\Apache.Arrow.csproj", "{BA6B2B0D-EAAE-4183-8A39-1B9CF571F71F}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Apache.Arrow.Tests", "test\Apache.Arrow.Tests\Apache.Arrow.Tests.csproj", "{9CCEC01B-E67A-4726-BE72-7B514F76163F}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{BA6B2B0D-EAAE-4183-8A39-1B9CF571F71F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{BA6B2B0D-EAAE-4183-8A39-1B9CF571F71F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{BA6B2B0D-EAAE-4183-8A39-1B9CF571F71F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{BA6B2B0D-EAAE-4183-8A39-1B9CF571F71F}.Release|Any CPU.Build.0 = Release|Any CPU
+		{9CCEC01B-E67A-4726-BE72-7B514F76163F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{9CCEC01B-E67A-4726-BE72-7B514F76163F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{9CCEC01B-E67A-4726-BE72-7B514F76163F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{9CCEC01B-E67A-4726-BE72-7B514F76163F}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {FD0BB617-6031-4844-B99D-B331E335B572}
+	EndGlobalSection
+EndGlobal

csharp/src/Apache.Arrow/Apache.Arrow.csproj

@@ -1,9 +1,11 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <Import Project="../../build/Common.props" />
 
   <PropertyGroup>
     <TargetFramework>netstandard1.3</TargetFramework>
+    <LangVersion>7.2</LangVersion>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <Authors>Apache</Authors>
     <Product>Apache Arrow library</Product>
     <Copyright>2018 Apache Software Foundation</Copyright>
@@ -15,16 +17,6 @@
     <Version>0.0.1</Version>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
-    <LangVersion>7.2</LangVersion>
-    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-  </PropertyGroup>
-
-  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
-    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-    <LangVersion>7.2</LangVersion>
-  </PropertyGroup>
-
   <ItemGroup>
     <PackageReference Include="System.Buffers" Version="4.5.0" />
     <PackageReference Include="System.Memory" Version="4.5.1" />

dev/release/rat_exclude_files.txt

@@ -164,6 +164,7 @@ c_glib/gtk-doc.make
 csharp/.gitattributes
 csharp/src/Apache.Arrow/Flatbuf/*
 csharp/build/Common.props
+csharp/Apache.Arrow.sln
 csharp/src/Apache.Arrow/Apache.Arrow.csproj
 csharp/src/Apache.Arrow/Properties/Resources.Designer.cs
 csharp/src/Apache.Arrow/Properties/Resources.resx