AVRO-4187: [csharp] fix bug causing csharp Avrogen generated "SCHEMA" property to contain csharp instead of avro namespaces when using --namespace flag

[DavidNeukamXXXLutz]

What is the purpose of the change

This PR fixes the issue AVRO-4187

When using avrogen for generating C# models from a schemafile, the --namespace option doesn't behave as expected.
When running 

avrogen -s file.avsc . --namespace my.avro.ns:my.csharp.ns

the namespace within the SCHEMA property of the generated C# files is changed to my.csharp.ns, but should be left with the value my.avro.ns.

public partial class TestModel : ISpecificRecord
{
	public static Schema _SCHEMA = Avro.Schema.Parse("{\"type\":\"record\",\"name\":\"TestModel\",\"namespace\":\"my.avro.ns\",\"fields\":[{\"name\":\"e" +
			"ventType\",\"type\":{\"type\":\"enum\",\"name\":\"EventType\",\"namespace\":\"my.avro.ns\",\"sym" +
			"bols\":[\"CREATE\",\"UPDATE\",\"DELETE\"]}}]}");
	private my.csharp.ns.EventType _eventType;
           // More generated code
    }

Verifying this change

This change added tests and can be verified as follows:

• Extended existing namespace tests to assert that the avro namespace is contained in the SCHEMA property
• Extended existing namespace tests to assert that the csharp namespace is not contained in the SCHEMA property

Documentation

• Does this pull request introduce a new feature? no

[DavidNeukamXXXLutz]

I identified an issue with this change.
Though being a correct fix, people could currently rely on the buggy behaviour of having the C# names in the SCHEMA files.
For them, this could be a breaking change.

Another issue is, that the Avro deserializer does not support mapping form Avro Namespaces to C# namespaces.
The issue is, that the ObjectCreator does not have an extension point to provide the (reverse) namespace mappings.
It searches for the types with the namespaces given in the Avro schema.
By a hacky solution, I could make this work by adding the types to typeCacheByName via reflection.

I checked the Java implementation and it looks like they also dont support namespace mapping.

I thend to come to the conclusion, that the avrogen should not have a --namespace flag as the other Avro tooling does not support namespace mappings - so we are depending on a bug in the code generation to be able to deserialize the types again (affecting the contract).

I would suggest deprecating the --namespace flag with that information.