Add a real regex condition instead of relying on callback

Problem/Motivation

As described in #3263910: Credit for Add a process plugin to skip on regular expression, the callback condition can be used to handle regex. It's really clumsy though and I finally got the motivation to add a dedicated matches condition that uses regex.

Proposed resolution

Add a matches condition that uses the following syntax:

 process:
   has_a_digit:
     plugin: evaluate_condition
     condition:
       plugin: matches
       regex: '/\d+/'
     source: source_string

It should validate that regex exists and is a valid regex string in the constructor. It should validate that the source is a string during migration.

User interface changes

N/A

API changes

New migration_condition plugin available!

Data model changes

N/A