[GitHub] Add exclude.size property to the config

[brendan-kellam]

This PR adds the ability to exclude GitHub repositories based on their disk usage. E.g.,:

{
    "$schema": "./schemas/v2/index.json",
    "repos": [
        {
            "type": "github",
            "repos": [
                "sourcebot-dev/sourcebot"
            ],
            "exclude": {
                "size": {
                    "min": 16000, // 16 KB
                    "max": 1000000000 // 1GB
                }
            }
        }
    ]
}