Audit log data structures.
The audit log is logging that is produced by user actions and consumed
by users in the form of the
mz_catalog.mz_audit_events SQL table and
by the cloud management layer for billing and introspection. This crate
is designed to make the production and consumption of the logs type
safe. Events and their metadata are versioned and the data structures
replicated here so that if the data change in some other crate, a
new version here can be made. This avoids needing to poke at the data
when reading it to determine what it means and should have full backward
compatibility. This is its own crate so that production and consumption can
be in different processes and production is not allowed to specify private
data structures unknown to the reader.