SQL source plugins: allow defining conditions and join in migration yml

I know this is both old and has up-in-the-air elements to it, BUT, I successfully applied MR #849 to a Drupal 10.1 site, and then successfully migrated users of just one role. (I only applied the changes in MR #849, I didn't grab anything from #2833060 🐛 SqlBase::prepareQuery() should be called also on count Needs work .)

I did get warnings when I ran drush mim, but it still ran successfully -- but I'll share them anyway:

 [warning] Undefined array key "conditions" SqlBase.php:365
 [warning] foreach() argument must be of type array|object, null given SqlBase.php:365
 [warning] Undefined array key "joins" SqlBase.php:370
 [warning] foreach() argument must be of type array|object, null given SqlBase.php:370
 [warning] Undefined array key "fields" SqlBase.php:375
 [warning] foreach() argument must be of type array|object, null given SqlBase.php:375
 [warning] Undefined array key "conditions" SqlBase.php:365
 [warning] foreach() argument must be of type array|object, null given SqlBase.php:365
 [warning] Undefined array key "joins" SqlBase.php:370
 [warning] foreach() argument must be of type array|object, null given SqlBase.php:370
 [warning] Undefined array key "fields" SqlBase.php:375
 [warning] foreach() argument must be of type array|object, null given SqlBase.php:375

My very rough take on those warnings is that they mean my conditions syntax could/should be better. (I copied from one of the code examples in SqlBase.php, but maybe I certainly may have misinterpreted it.)

Speaking of which, here's the "source" section of my user migration YML:

source:
  plugin: d7_user
  joins:
    -
      table: users_roles
      alias: ur
      condition: u.uid = ur.uid
  conditions:
    -
      field: ur.rid
      value: [6]
      operator: IN
  distinct: TRUE