Clarify license of buf.build assets

[luisgerhorst]

Would it be possible to add a license as recommended by https://buf.build/docs/bsr/module/manage/#module-documentation ? I guess the default would be that the assets are under the same license as Parca, however, common license checkers do not appear to detect this properly when a Go module uses the buf.build asset as a dependency.

[metalmatze]

Interesting, wasn't aware of that so far. I think we can safely copy the LICENSE from the root to the proto/ folder.