Place a dedicated `Expr` type between datafusion and protobuf. While
this type is currently only used during serialization, it will be used
within `Predicate` in a follow-up change to enable de-duplication of
predicates and avoid the double parsing of datafusion expressions (we
are already somewhat parsing them when we check for valid delete
predicates).
This change looks larger than it is. In practice it just separates the
conversion "datafusion => protobuf" into "datafusion => IOx => protobuf"
and "protobuf => datafusion" into "protobuf => IOx => datafusion". So
(apart from the error types) this is functionally the same.
Two reasons:
1. I wanna decouple `parquet_file` from `query` (nearly done, needs a
small follow-up PR).
2. `predicate` will have more and more features (like serialization)
which justifies a new home
Marco Neumann
Nga Tran
Andrew Lamb