[FEATURE REQ] Fluent Strongly Typed Context

We have two problems with C# sdk
1- It is not strongly typed
2- It is unreadable, long methods and too much attributes

Suggestion suppose people to put Activities in interface or class. the the IDurableOrchestrationContext should be able to produce proxy for strongly typed methods

let's instead of

```csharp
class ActivityClass
{
[FunctionName(nameof(MethodName))]
public Task MethodName(int x)
}
//orchestrator
context.CallActivityWithRetryAsync(nameof(ActivityClass.MethodName), retryOptions, input);
```
We should be able to do:
```csharp
class ActivityClass
{
public Task MethodName(int x)
}
//orchestrator
context.ClientOf().WithRetry(retryOptions).MethodName(10);
```

Will be great

Start by reading IDurableOrchestrationContext and the existing CallActivityWithRetryAsync usage shown in the issue. Identify how activities are currently represented and invoked, then define the scope and completion criteria for a strongly typed ClientOf proxy with retry support and method invocation matching the example.

Written by the indexing model from the issue text.