Toolprovider: config and validation

[ofalvai]

Checklist

• I've read and followed the Contribution Guidelines
• README.md is updated with the changes (if needed)

Version

Requires a MAJOR/MINOR/PATCH version update

Context

First commit of upstreaming https://github.com/bitrise-io/toolprovider to this repo.

Changes

• Define basic YML config structure
• Implement config validation

Investigation details

Decisions