buildkitd: Add ability to restrict frontends and gateway sources
When building images on trusted runners we want to ensure that only the Blubber frontend is used. This commit adds the ability to restrict the frontends and gateway sources in buildkitd.
buildkit.toml entries that would be use on trusted runners:
allowed-frontends = ["gateway.v0"] allowed-gateway-sources = ["docker-registry.wikimedia.org/repos/releng/blubber/buildkit"]
Bug: T329220