[Feature Request] More agent and pool verbs (?)

**Is your feature request related to a problem? Please describe.**
Ease of use, expected it to be there, feature incomplete.

**Describe the solution you'd like**
```
az pipelines agent delete --pool-id --agent-id
az pipelines agent enable --pool-id --agent-id
az pipelines agent disable --pool-id --agent-id
az pipelines pool delete --pool-id
az pipelines pool create --name
```

**Additional context**
Related to #853, #808 and [this one](https://github.com/Azure/azure-cli-extensions/issues/1273).