Require `*Definition` methods to return `\GraphQL\Language\Source|NULL`

We currently allow returning `string|NULL` from methods such as `getSchemaDefinition` and `getExtensionDefinition`. However, that does not allow providing location information which is very useful in debugging.

To ensure SDL can always be traced back to its origins we should force plugins to return a `Source` instance. The only required value for `Source` is the actual string, but it's very easy to provide a name such as "Automatically Generated by {plugin ID}".

Since this is a change of return type for something that's implemented in user land it's a breaking change and can only be done in 5.x.