Rename workflowagents package to workflowagent

ADK will introduce a `workflowagent` in the future, its code can easily fit into the `workflowagent`. Plural package names is an anti pattern in Go because the package name is a part of the identifier in code. Consider the difference between before and after:

Before:

```go
workflowagents.New(...)
```

After:
```go
workflowagent.New(...)
```

Rename workflowagents to workflowagent to avoid breaking changes in the future.