Add a new pg_walinspect function to extract FPIs from WAL records

Hi,

Here's a patch that implements the idea of extracting full page images
from WAL records [1] [2] with a function in pg_walinspect. This new
function accepts start and end lsn and returns full page image info
such as WAL record lsn, tablespace oid, database oid, relfile number,
block number, fork name and the raw full page (as bytea). I'll
register this in the next commitfest.

Thoughts?

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=d497093cbecccf6df26365e06a5f8f8614b591c8
[2] https://postgr.es/m/CAOxo6XKjQb2bMSBRpePf3ZpzfNTwjQUc4Tafh21=jzjX6bX8CA@mail.gmail.com

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com