Add BUSL 1.1 license

This [BUSL 1.1 is an SPDX license](https://spdx.org/licenses/BUSL-1.1.html) and I think originally came from https://mariadb.com/bsl11/.

I was going to make a PR to add an ler file for this but I’m sure what should count as a valid BUSL-1.1 license.

For example, I was looking into [github.com/hashicorp/terraform's license](https://github.com/hashicorp/terraform/blob/main/LICENSE) and it seems to have the bulk of the BUSL-1.1 license but not the “Standard License Header” and “Covenants of Licensor”. Both sections appear on the SPDX license text page.

Does licensecheck have a process to define what counts as a valid license?