feature: functions: create function triggers with doctl

**What is the problem this feature would solve? Please describe.**

I would like to be able to create Serverless Function Triggers with `doctl` and not `project.yml`.

**Describe the solution you'd like**

```bash
doctl serverless trigger create --name "trigger" --cron "* * * * * *" --payload "{}"
```
**Additional context**

I would like to be able to automate the creation of triggers without needing to update the `project.yml`