config: move RequiresBilling to sdk.yaml

[zhumin8]

RequiresBilling should not be a language specific properly, should move to sdk.yaml eventually.

Currently used for .repo-metadata.json, but not populated by internal/repometadata/repometadata.go.

In Java, it is an optional field in generation_config.yaml for hermetic_build (e.g. here). So if we have this value in sdk.yaml, Java might still use some override values to reduce diffs for initial migration.

[jskeet]

Do we actually think it's useful anyway? I strongly suspect that this field in particular isn't used.