[ethereum/go-ethereum] crypto/bn256: add missing license file, release wrapper in BSD-3 (#17451)

We missed the `LICENSE` file while vendoring in the Cloudflare bn256 lib. Fixed in this PR.

This PR also changes the license of the Cloudflare/Google implementation switcher code from LGPLv3 to BSD-3. I didn’t feel it fair to restrict the massive bn256 crypto code with 40 lines of wrapper that we added.
You can view, comment on, or merge this pull request online at:

https://github.com/ethereum/go-ethereum/pull/17451

— Commit Summary —

* crypto/bn256: add missing license file, release wrapper in BSD-3

— File Changes —

A crypto/bn256/LICENSE (28)
M crypto/bn256/bn256_fast.go (18)
M crypto/bn256/bn256_fuzz.go (18)
M crypto/bn256/bn256_slow.go (18)
A crypto/bn256/cloudflare/LICENSE (27)

— Patch Links —

https://github.com/ethereum/go-ethereum/pull/17451.patch
https://github.com/ethereum/go-ethereum/pull/17451.diff