Config schema for effects are missing

Adding config schema for the convert image effect. There's plenty more of these so not setting this to "needs review".

As the image.effect.image_convert config schema is defined by the image.module therefore this module cannot override it because that still leads to config schema errors:

>  [error]  Schema errors for image.style.open_graph_image with the following errors: image.style.open_graph_image:effects.3326f4a4-7222-4e7d-93f6-0b7c3f338050.data.extension missing schema 

The definition of the extension property gets removed by the patch #2.

I added some schemas to image_blur and image_composite effects

I commited some schemas to image_blur and image_composite effects. So, we need to add other schemas

kewl! This is fixed, now only crediting is missing and issue can be closed.

This is not fully fixed. See comment #13

I have now added config schema definitions for all image effects.
Configuration forms for image effects could also be improved upon, e.g. to property mark required fields, define numeric ranges, etc..
These improvements could then also be mirrored via config validation in the schema.yml file.

However, that seems like something to be tackled in a different issue.
For now I have only added config validation, where it currently exists in the configuration forms, to make sure the form input and config data is in sync validation-wise.

Adding the MR diff as patch file, for Composer-based patching workflows.

@svendecabooter awesome! Merged!