AVRO-2156: Map Avro namespace to C# namespaces during code generation

[blachniet]

AVRO-2156

Added new optional command line argument, --namespace <my.avro.ns:my.csharp.ns>.

Normally, avrogen.exe uses the namespaces defined in the schema or protocol as the namespace in the generated C# code. This default behavior has not changed as a part of this pull request. However, if the user wishes to replace the Avro namespace with a C#-friendly namespace in the generated code, they can provide the --namespace option.

For example, if this is our protocol:

// sample.avdl
@namespace("org.example.sample")
protocol MySampleProtocol {
    fixed MD5(16);
}

avrogen.exe -p sample.avdl output would produce something like:

// MD5.cs
namespace org.example.sample
{
    public partial class MD5 : SpecificFixed //...
}

If the user wanted a more C#-friendly namespace in the generated code, the could run avrogen.exe -p sample.avdl output --namespace "org.example.sample:Sample.Models" to produce something like:

// MD5.cs
namespace Sample.Models
{
    public partial class MD5 : SpecificFixed //...
}

I am open to any suggestions, whether implementation details or command line interface. Thanks!