Consider creating separate C# decorator package to avoid dependency issues

With the new dynamicModel decorator, if we start adding this to Azure specs, we will need to make the rest specs repo depend on the unbranded emitter. This means we would need to upgrade the unbranded emitter as soon as a new TypeSpec version is released which would vastly accelerate our current SLA. An alternative would be to have a separate package that holds any decorators that we are shipping as this would be much easier to upgrade.