Ensure only selected fields are serialized

Problem/Motivation

Whenever you save the Processors page, the search_api_glossary processor plugin keeps adding all new fields into the config, whereas it should only serialize and save relevant fields.

Steps to reproduce

Add two new fields and then resave the processors page, the config will be updated with new serialized data unnecessarily.

Proposed resolution

Filter the config before saving.

Remaining tasks

Provide issue fork/patch.

User interface changes

N/A