Eliminate dependency from pageserver_api to postgres_ffi (#12273)

Introduce a separate `postgres_ffi_types` crate which contains a few
types and functions that were used in the API. `postgres_ffi_types` is a
much small crate than `postgres_ffi`, and it doesn't depend on bindgen
or the Postgres C headers.

Move NeonWalRecord and Value types to wal_decoder crate. They are only
used in the pageserver-safekeeper "ingest" API. The rest of the ingest
API types are defined in wal_decoder, so move these there as well.

libs/wal_decoder/src/models.rs

@@ -25,6 +25,9 @@
 //!                     |
 //!                     |--> write to KV store within the pageserver
 
+pub mod record;
+pub mod value;
+
 use bytes::Bytes;
 use pageserver_api::reltag::{RelTag, SlruKind};
 use postgres_ffi::walrecord::{