[Spec] CosmosDBTriggerAttribute to take configuration key

Instead of having configurations hardcoded in CosmosDBTriggerAttribute, we should be able to pass the parameters through configurations.

Something like this

`[CosmosDbTrigger("MyTriggerConfig")] ... `

Then in the appSettings.json

```
"MyTriggerConfig" : {

"Database": "MyDb",
"Container" : "MyContainer",
"Lease" : "MyLease",
// . . .
}
```

Guess that can apply on other triggers as well.