Proto.Actor Activities

### Description
An activity type provider that uses a given (set of) protobuf file(s) containing service definitions would allow users to orchestrate actors by using these activities in a workflow.

### Example

For example, take the following virtual actor definitions:

```proto
syntax = "proto3";

service ThermostatGrain {
rpc Read () returns (Temperature);
rpc Update (Temperature) returns (Empty);
}

service FireplaceGrain {
rpc Start () returns (Empty);
rpc Stop () returns (Empty);
}
```

The activity type provider would automatically yield the following activities using appropriate conventions:

- Read Thermostat
- Update Thermostat
- Start Fireplace
- Stop Fireplace

Using these activities, one could write the following workflow that keeps the room at a comfortable temperature:

```csharp
class ComfyRoomWorkflow : IWorkflow
{
public void Build(IWorkflowDefinitionBuilder workflow)
{
var temperatureVariable = new Variable();
const desiredTemperature = new Temperature(21);

workflow.WithRoot(new Sequence
{
Variables = { temperatureVariable }
Activities =
{
// Every 5 minutes.
Timer.FromMinutes(5),

// Read the current temperature into a variable.
new ReadThermostat(temperatureVariable ),

// If the current temprerature drops below a certain threshold:
new If(context => temperatureVariable.Get(context) < desiredTemperature)
{
// Then heat things up a little.
Then = new StartFireplace(),

// Else, stop the fire place. May want to check its current state first, but leaving that out for simplicity.
Else = new StopFireplace()
}
}
})
}
}
```