Add bird3 extension

[lucasvbeek]

Just checking if you'd accept this as a new extension, I'd be happy to contribute it in a PR.

[smira]

Converted to a discussion, as this is a question.

Please tell us more about it - what is required, why is it better, can we remove some extension eventually?

[lucasvbeek]

As far as I know it's mostly a huge internal refactor that includes multithreaded route processing and modernises the codebase. It's not strictly required, but for new setups I prefer using the most recent major version to avoid unnecessary potential tech debt.

Packaging should be mostly s/bird2/bird3, so not a large amount of effort to create and maintain. I'd keep bird2 alongside it for the foreseeable future as it's still relatively new.

[smira]

if it builds easily, I don't see why not - let's support it, we probably need good README explaining why would one prefer bird2 or bird3