secrets.yml support

[ofalvai]

Is your feature request related to a problem? Please describe.

I don't want to hardcode my MQTT username and password in my local.yml file. I don't see any option to pass these values from an env var or secret file that I can gitignore.

Describe the solution you'd like

Something similar to what Home Assistant and Zigbee2MQTT do with the special !secret username YAML syntax. The values come from a secrets.yaml file in the same folder that can be gitignored.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.