Fetch generator

[a4z]

Start with :
Add script to get a generator from a pull request in the djinni-generator repo

More to come, The conan download needs to be replaced,
and I think getting the generator via cmake download a (pre) release is a good idea

We might commit this script to enable that feature for #76 sooner.
Needs some adoption in the CI

[a4z]

Ok, this is stupid. Technically, it's possible to fetch the generator build of a PR from the generator repo.
Because GitHub requires authentication for doing this, we need to use secrets to pass an access token.
But, for PRs from a fork, access to secure tokens is not possible due to security reasons 😭

So the simple download of a PR pull request artifact, and thus for a wanted generator other than the latest release or the latest pre-release, is currently not possible.