Remove meta data of PGLZ symbolized by PGLZ_Header

PGLZ_Header is being used to store some meta data related to the data
being compressed like the raw length of the uncompressed record or some
varlena-related data, making it unpluggable once PGLZ is stored in
src/common as it contains some backend-only code paths with the management
of varlena structures. The APIs of PGLZ are reworked at the same time to
do only compression and decompression of buffers without the meta-data
layer, simplifying its use for a more general usage.

On-disk format is preserved as well, so there is no incompatibility with
previous major versions of PostgreSQL for TOAST entries.

This step is necessary for FPW compression in WAL that will use PGLZ this
way.