pgsql: Add PL/Sample to src/test/modules/

Add PL/Sample to src/test/modules/

PL/Sample is an example template of procedural-language handler.  This
can be used as a base to implement a custom PL, or as a facility to test
APIs dedicated to PLs.  Much more could be done in this module, like
adding a simple validator, but this is left as future work.

The documentation included originally some C code to understand the
basics of PL handler implementation, but it was outdated, and not really
helpful either if trying to implement a new procedural language,
particularly when it came to the integration of a PL installation with
CREATE EXTENSION.

Author: Mark Wong
Reviewed-by: Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/20200612172648.GA3327@2ndQuadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/adbe62d04b360bbd408d97e447932d8078485972

Modified Files
--------------
doc/src/sgml/plhandler.sgml                     |  60 +-------
src/test/modules/Makefile                       |   1 +
src/test/modules/plsample/.gitignore            |   3 +
src/test/modules/plsample/Makefile              |  20 +++
src/test/modules/plsample/README                |   6 +
src/test/modules/plsample/expected/plsample.out |  36 +++++
src/test/modules/plsample/plsample--1.0.sql     |  14 ++
src/test/modules/plsample/plsample.c            | 183 ++++++++++++++++++++++++
src/test/modules/plsample/plsample.control      |   8 ++
src/test/modules/plsample/sql/plsample.sql      |  15 ++
10 files changed, 290 insertions(+), 56 deletions(-)