Implement SQL-level LFC prewarming through sideloading pages

We allow extraction and loading of LFC contents through SQL-callable
functions.

The LFC sideloading is based on these principles:

- While sideloading, the page won't get evicted (e.g. through chunk recycling)
- Access to existing LFC contents won't be negatively impacted by sideloading
- New writes to LFC may block, but only while sideloading is ready to
  write to the chunk.
- prewarming/sideloading only adds data to the LFC, it won't evict pages.

Co-authored-by: Matthias van de Meent <matthias@neon.tech>
Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/neon/Makefile

@@ -6,6 +6,7 @@ OBJS = \
 	$(WIN32RES) \
 	extension_server.o \
 	file_cache.o \
+	file_cache_prewarm.o \
 	hll.o \
 	libpagestore.o \
 	logical_replication_monitor.o \
@@ -34,6 +35,8 @@ DATA = \
 	neon--1.2--1.3.sql \
 	neon--1.3--1.4.sql \
 	neon--1.4--1.5.sql \
+	neon--1.5--1.6.sql \
+	neon--1.6--1.5.sql \
 	neon--1.5--1.4.sql \
 	neon--1.4--1.3.sql \
 	neon--1.3--1.2.sql \