Create CircleCI Orbs to notify slack to extend a lifetime of a GCP instance

[zoetrope]

What

The step to notify slack is used in multiple repositories.

• https://github.com/cybozu-go/neco/blob/f179c654aaee8042e3e0114ea296fecaf82e63d9/.circleci/config.yml#L62
• https://github.com/cybozu-go/neco-apps/blob/49adaf249896d17049297d023092bf191c83ec20/.circleci/config.yml#L79

This step is the same in those repositories, so it should be shared using CircleCI Orbs.

Also, in the step, I want to send a mention to the corresponding user if possible.

How

Describe how to address the issue.

Checklist

• [ ] Finish implementation of the issue
• [ ] Test all functions
• [ ] Have enough logs to trace activities
• [ ] Notify developers of necessary actions