[META] Document available configuration properties in process plugins

Problem/Motivation

Some process plugins does not document available configuration keys in the docblock section. It makes it harder to understand plugin behaviour, and you have to go through examples or look at the ::transform method.

At least the following plugins are affected:

• DefaultEntityValue.php
• EntityGenerate.php
• EntityLookup.php
• EntityValue.php
• FileBlob.php
• Service.php
• StrReplace.php

Proposed resolution

1) Create a child issue for each one;
2) Submit a merge request with the documentation changes;

See #2974206: EntityLookup doesn't document configuration properties.