Adds an optional, repeatable parameter block to the postgresql_role resource, which is used to define arbitary configuration parameters for the role. This is equivalent to using ALTER ROLE [role] SET [param] TO [value].

Example:

resource "postgresql_role" "test_role" {
  name = "audited_user"

  parameter {
    name  = "pgaudit.log"
    value = "all"
  }
}

This is more or less an alternate implementation to #211, and is in line with the suggestion in a comment on that PR.

Some implementation notes:

1. Some configuration parameters are already supported by dedicated arguments: search_path, statement_timeout, idle_in_transaction_timeout, and role. To prevent those existing arguments from conflicting with this one and causing perpetual diffs, those parameters cannot be set with a parameter block.

2. There are some peculiarities with value quoting:

   Most general configuration parameters, for example client_min_messages, can either be quoted string literals or bare tokens in the ALTER ROLE statement. At least one — search_path — cannot be quoted. Or rather, quoting will give you unexpected results. Others, for instance pgaudit.log, must be quoted, or the statement will produce an error.

   To accommodate all circumstances including ones I wasn't able to personally test, I opted to include an optional quote property in the parameter block that defaults to true, so quoting can be selectively disabled if required.

Closes #210