pageserver: move things around to prepare for decoding logic (#9504)

## Problem

We wish to have high level WAL decoding logic in `wal_decoder::decoder`
module.

## Summary of Changes

For this we need the `Value` and `NeonWalRecord` types accessible there, so:
1. Move `Value` and `NeonWalRecord` to `pageserver::value` and
`pageserver::record` respectively.
2. Get rid of `pageserver::repository` (follow up from (1))
3. Move PG specific WAL record types to `postgres_ffi::walrecord`. In
theory they could live in `wal_decoder`, but it would create a circular
dependency between `wal_decoder` and `postgres_ffi`. Long term it makes
sense for those types to be PG version specific, so that will work out nicely.
4. Move higher level WAL record types (to be ingested by pageserver)
into `wal_decoder::models`

Related: https://github.com/neondatabase/neon/issues/9335
Epic: https://github.com/neondatabase/neon/issues/9329

libs/postgres_ffi/src/lib.rs

@@ -217,6 +217,7 @@ macro_rules! enum_pgversion {
 
 pub mod pg_constants;
 pub mod relfile_utils;
+pub mod walrecord;
 
 // Export some widely used datatypes that are unlikely to change across Postgres versions
 pub use v14::bindings::RepOriginId;