Logging query parmeters in auto_explain

Hi hackers,

Inspired by a question on IRC, I noticed that while the core statement
logging system gained the option to log statement parameters in PG 13,
auto_explain was left out.

Here's a patch that adds a corresponding
auto_explain.log_parameter_max_length config setting, which controls the
"Query Parameters" node in the logged plan.  Just like in core, the
default is -1, which logs the parameters in full, and 0 disables
parameter logging, while any other value truncates each parameter to
that many bytes.

- ilmari