[PostgREST] Add support for calling rpc functions with filters

# Feature request

Support appending a filter after an `rpc`, example:

```swift
struct Country: Decodable {
  let id: Int
  let name: String
}

let country: Country = await supabase.database
  .rpc("list_stored_countries")
  .eq("id", value: 1) // THIS
  .single()
  .execute()
  .value
```