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basics of PL handler implementation, but it was outdated, and not really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