[SchemaRegistry] rename namespace + package names

[swathipil]

• investigate process to change/register namespace for serializer + async serializer + package names
• CONFIRM WITH JOHAN:
  rename package name: azure-avroserializer (?) or azure-schemaregistry-avroserializer(?)

rename namespace for serializer: azure.schemaregistry.serializer.avroserializer.AvroSerializer

async serializer namespace: azure.schemaregistry.serializer.avroserializer.aio.AvroSerializer

[yunhaoling]

nspkg: azure.schemaregistry.serializer

[swathipil]

"schemaregistry is acting as namespace package, so don't need nspkg" -> clarify with Johan