fix(codegen): Creates multiple steps for serialization source generator (#628)

Roslyn Source generators are not supposed to access I/O. To get around this we have to use `AdditionalFiles` to give the analyzer access to read/load the schema files.
To write the schema files we have to use a separate program altogether.

- [X] Adds SerializationSchemaGenerator
- [X] Splits out the SerializationGenerator code

Projects/SerializationGenerator/SourceGeneration/SourceGeneration.Attribute.cs

@@ -17,7 +17,7 @@ using System.Collections.Immutable;
 using System.Text;
 using Microsoft.CodeAnalysis;
 
-namespace SerializationGenerator
+namespace SourceGeneration
 {
     public static partial class SourceGeneration
     {