Include `LICENSE` file in the module on `buf push`

[cyinma]

fixes BSR-575

This is to also push the LICENSE file that relative to the root of the module on buf push, so that we can determine the license of a BSR repository based on the license in its latest main commit.

For license file name that we accept, we might want to also accept other names (e.g. LICENSE.md, LICENSE.txt), but I think we can just start with just the LICENSE for now