Move GenerateSerializerType into MagicOnionClientGenerationAttribute

[mayuki]

Fixes #738

GenerateSerializerType remains after compiling, which causes conflicts with InternalsVisibleTo.
Accordingly, GenerateSerializerType has been moved under MagicOnionClientGenerationAttribute, which is a breaking change.

Before

[MagicOnionClientGeneration(typeof(IGreeterService), Serializer = GenerateSerializerType.MemoryPack)]
partial class MagicOnionInitializer {}

After

[MagicOnionClientGeneration(typeof(IGreeterService), Serializer = MagicOnionClientGenerationAttribute.GenerateSerializerType.MemoryPack)]
partial class MagicOnionInitializer {}