Add functions.sh and allowed commands

Support for postUpgradeTasks on packages, initially introducing changelog_bump only which will generate a changelog update and bump either the major, minor, patch, or bb version with a general description entry.

Example for implementation in the customer template (small snippet with just the package rule):

{
  "packageRules": [
    {
      "matchDatasources": ["git-tags"],
      "postUpgradeTasks":  {
        "commands": [
          "source functions.sh && changelog_bump -v minor"
        ],
        "fileFilters": ["CHANGELOG.md"],
        "executionMode": "branch"
      }
    }
  ]
}