autho_explain: Add GUC to log query parameters

auto_explain.log_parameter_max_length is a new GUC part of the
extension, similar to the corresponding core setting, that controls the
inclusion of query parameters in the logged explain output.

More tests are added to check the behavior of this new parameter: when
parameters logged in full (the default of -1), when disabled (value of
0) and when partially truncated (value different than the two others).

Author: Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/87ee09mohb.fsf@wibble.ilmari.org
