ARROW-4543: [C#] Update Flat Buffers code to latest version

Update FlatBuffers code to latest version.
Mark all FlatBuffer types as internal.

Author: eerhardt

csharp/src/Apache.Arrow/Flatbuf/Block.cs

@@ -8,7 +8,7 @@ namespace Apache.Arrow.Flatbuf
 using global::System;
 using global::FlatBuffers;
 
-public struct Block : IFlatbufferObject
+internal struct Block : IFlatbufferObject
 {
   private Struct __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

csharp/src/Apache.Arrow/Flatbuf/Buffer.cs

@@ -10,7 +10,7 @@ namespace Apache.Arrow.Flatbuf
 
 /// ----------------------------------------------------------------------
 /// A Buffer represents a single contiguous memory segment
-public struct Buffer : IFlatbufferObject
+internal struct Buffer : IFlatbufferObject
 {
   private Struct __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

csharp/src/Apache.Arrow/Flatbuf/DictionaryBatch.cs

@@ -14,7 +14,7 @@ namespace Apache.Arrow.Flatbuf
 /// There is one vector / column per dictionary, but that vector / column
 /// may be spread across multiple dictionary batches by using the isDelta
 /// flag
-public struct DictionaryBatch : IFlatbufferObject
+internal struct DictionaryBatch : IFlatbufferObject
 {
   private Table __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

csharp/src/Apache.Arrow/Flatbuf/DictionaryEncoding.cs

@@ -10,7 +10,7 @@ namespace Apache.Arrow.Flatbuf
 
 /// ----------------------------------------------------------------------
 /// Dictionary encoding metadata
-public struct DictionaryEncoding : IFlatbufferObject
+internal struct DictionaryEncoding : IFlatbufferObject
 {
   private Table __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

csharp/src/Apache.Arrow/Flatbuf/Enums/DateUnit.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum DateUnit : short
+internal enum DateUnit : short
 {
  DAY = 0,
  MILLISECOND = 1,

csharp/src/Apache.Arrow/Flatbuf/Enums/Endianness.cs

@@ -7,7 +7,7 @@ namespace Apache.Arrow.Flatbuf
 
 /// ----------------------------------------------------------------------
 /// Endianness of the platform producing the data
-public enum Endianness : short
+internal enum Endianness : short
 {
  Little = 0,
  Big = 1,

csharp/src/Apache.Arrow/Flatbuf/Enums/IntervalUnit.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum IntervalUnit : short
+internal enum IntervalUnit : short
 {
  YEAR_MONTH = 0,
  DAY_TIME = 1,

csharp/src/Apache.Arrow/Flatbuf/Enums/MessageHeader.cs

@@ -13,7 +13,7 @@ namespace Apache.Arrow.Flatbuf
 /// Arrow implementations do not need to implement all of the message types,
 /// which may include experimental metadata types. For maximum compatibility,
 /// it is best to send data using RecordBatch
-public enum MessageHeader : byte
+internal enum MessageHeader : byte
 {
  NONE = 0,
  Schema = 1,

csharp/src/Apache.Arrow/Flatbuf/Enums/MetadataVersion.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum MetadataVersion : short
+internal enum MetadataVersion : short
 {
   /// 0.1.0
  V1 = 0,

csharp/src/Apache.Arrow/Flatbuf/Enums/Precision.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum Precision : short
+internal enum Precision : short
 {
  HALF = 0,
  SINGLE = 1,