Heh, so, this req is actually because I thought it was a bit klunky to switch to a newer TF, with that packed-string field. [this change](https://github.com/pantsbuild/pants/pull/15950) which just got merged adds a codec for encoding/decoding to the known_versions packed string, so it's at least easier to generate them. I'm not sure how much of a hurdle it would be to accept a Union[str, ExternalToolVersion] for that field (or to drop the str altogether, breaking backwards compatibility). I've got another partially-baked change that scrapes all of the known_versions for TF. I think having the known_versions defined and easy to switch to is a nice feature. We should probably omit the versions of TF we won't support (alphas, ancient versions).