Add process plugin gate_comparator

This has been created and will go out with the next release.

/**
 * Compares two values, and if the comparison is true, passes the source thru.
 *
 * @MigrateProcessPlugin(
 * id = "gate_comparator",
 * handle_multiples = TRUE
 * )
 *
 * Example usage: Compare value A to value B. If TRUE use the process pipeline
 * source.  If FALSE, use the 'when_false_value'.
 * @code
 * field_some_text_field:
 *   plugin: gate_comparator
 *   value_a: a string or number to compare or 'source' to use the source.
 *   comparison: = comparison to evaluate [=, <, >, !=, <=, >=]
 *   value_b: the other string or number to compare
 *   when_false_value: A value to use if the comparison is FALSE
 *   source:  The source value to use if the comparison is TRUE
 *
 * @endcode
 *
 * Example usage: Choosing the bigger value between two source fields.
 *
 * @code
 *  field_some_text_field:
 *   plugin: gate_comparator
 *   value_a: cost
 *   comparison: '>'
 *   value_b: average_cost
 *   when_false_value: average_cost
 *   source: cost
 *
 * @endcode
 */

Released with 8.x-2.8