Make the `field_config_base` structure fully validatable

Problem/Motivation

It would be really helpful to recipes if we could make fields and field storages fully validatable. This is a big lift; in many ways, it's the boss battle of config schema validation. So I think we should take it on in smaller, bite-sized chunks. Let's start with a base schema type that is used as the foundation of other config schema definitions: field_config_base.

Proposed resolution

Add constraints to as many properties of field_config_base as possible, and mark it fully validatable.