Missing config schema for field enhancers

Problem/Motivation

Missing configuration schema for "url_link" and "date_time_from_string" field enhancer plugins result in config schema warnings when importing configuration with these in use:

[warning] Schema errors for jsonapi_extras.jsonapi_resource_config.example with the following errors: jsonapi_extras.jsonapi_resource_config.example.field_example.enhancer.settings missing schema. These errors mean there is configuration that does not comply with its schema. This is not a fatal error, but it is recommended to fix these issues. For more information on configuration schemas, check out the documentation.

Proposed resolution

Any field enhancers that export settings to config need a schema defined, add config schema for the settings for these two field enhancer plugins.