Support generic Temporal transfer type converters

[tconley1428]

Problem

TemporalTransferTypePayloadConverter rejects converter types with open generic parameters. This prevents generators such as nexgen from registering transfer converters for generic protobuf-backed models.

Proposed behavior

Allow a matching open generic converter declaration:

[TemporalTransferTypeConverter(typeof(ModelTransferTypeConverter<>))]
public sealed class Model<T>
{
}

When serializing a concrete runtime type, or deserializing to a requested closed type, the SDK should:

1. Close the converter definition with the model type's generic arguments.
2. Apply the existing converter validation to that closed type.
3. Instantiate and cache the converter by closed model type.

Initial scope

The first version can require converter and model generic arity and argument order to match. That fully supports generated models.

A later converter-factory API could support hand-authored mappings whose converter generic shape differs from the model.