featureflag: Default `tx_restore_custom_hooks` on
Transactionality is being added to the SetCustomHooks
RPC via the
tx_restore_custom_hooks
feature flag. This RPC will be invoked with a
CLI client by GitLab administrators to set custom hooks for a
repository. Since this CLI originates the RPC invocations, feature flag
values are not propagated through the RPC context. This change default
enables the feature flag so tranactions can be enabled for the
set-hooks
CLI.