Add ability to register external commands/hooks to provide extra metadata

E.g.

- query external remotes for versions of dependencies
- ...

For that we might need to provide some layered mechanism to pick up configurations. E.g. in many datalad or even simpler git repo based cases it could go to `.git/config` or "datalad's config".

edit1: what I really wondered is even for those external hooks to provide

- augmentation of environment, e.g. ENV vars, so we could e.g. provide env var to tune git-annex commit message to git-annex branch, e.g. to include its version to help later troubleshooting odd changes etc.
- list env variables to include in the `_info.json:env` field

The issue names no files, tests, or entry points. Start by locating where command execution, configuration, and run metadata are collected, then determine how external hooks and environment variables would fit those paths. Done should include a decided configuration mechanism and documented behavior for dependency metadata, environment augmentation, and `_info.json:env`.

Written by the indexing model from the issue text.