feat: add method to merge (top-level) schemas

[tobikris]

We want to generate schema definitions for a type using generics.

Combining multiple parts by embedding is not working in this case though:

embedded field type cannot be a (pointer to a) type parameter (MisplacedTypeParam)

type CommonConfig struct {
	ServiceName string
}

type CombinedConfig[C any] struct {
	CommonConfig // this works
	C            // this does not
}

type CustomConfig struct {
	Foo string
}

This merge method achieves the same goal and seemed to be helpful in more than just our case.