Add missing source propery for skip by type pipeline [#3469310]

Problem/Motivation

In _migrate_skip_fields_instance_configuration there is a process pipeline that uses the migrate_skip_fields_by_type plugin. The process plugin requires a source configuration which is missing.

Proposed resolution

Add missing source configuration.

Issue fork migrate_skip_fields-3469310

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3469310-add-missing-source changes, plain diff MR !7
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