Customizing the CreateClient/CreateRetryableClient flow

`CreateClient`/`CreateRetryableClient` are used to create a client, that wraps invocation of methods as activities. This is achieved by creating proxies for retries (the `RetryProxy` class) and for the actual task scheduling (the `ScheduleProxy` class).

However, this flow is none-customizable - I have little control on how things gets invoked - I might want to write my own traces, send metrics, etc.

I'm looking at two possible solutions:

1. Expose a CreateClient method that accepts custom interceptors. Something along the lines of:
```cs
public virtual T CreateRetryableClient(RetryOptions retryOptions, bool useFullyQualifiedMethodNames, params IInterceptor[] additionalInterceptors) where T : class
{
if (!typeof(T).IsInterface && !typeof(T).IsClass)
{
throw new InvalidOperationException($"{nameof(T)} must be an interface or class.");
}

IInterceptor scheduleProxy = new ScheduleProxy(this, useFullyQualifiedMethodNames);
IInterceptor retryProxy = new RetryProxy(this, retryOptions);

T scheduleInstance = ProxyGenerator.CreateInterfaceProxyWithoutTarget(scheduleProxy);
T retriedInstance = ProxyGenerator.CreateInterfaceProxyWithTarget(scheduleInstance, retryProxy);
return ProxyGenerator.CreateInterfaceProxyWithTarget(retriedInstance, additionalInterceptors);
}
```
The downside being that we expose the implementation detail of `IInterceptor` to callers.
2. Expose the `RetryProxy` and `ScheduleProxy` classes as public (they are currently marked as internal) - thus allowing users to implement their own `CreateClient` method.
The downside being that this will result in duplicate code for consumers

Any suggestions? Better alternatives?