Add workflow to check for problems with certificates

[per1234]

If the certificates fail verification, a notification will be posted on the #team_tooling Slack channel.

If the certificates expire in less than 30 days, a notification will be posted on the #team_tooling Slack channel.

Demonstrations:

• Passing: https://github.com/arduino/arduino-lint/actions/runs/405355393
• Simulation of pending expiration: https://github.com/arduino/arduino-lint/actions/runs/405360814