feat: add convenience model validator

[danielgtaylor]

Adds a basic convenience utility for validating data against model schemas outside of the normal request/response flow.

Fixes #121.

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
validate.go	100.00%

📢 Thoughts on this report? Let us know!.

[nickajacks1]

Thanks, this is pretty much what I was looking for!

One other thing I realized yesterday (a bit unrelated) is that Schema and Registry have a cyclic dependency of sorts.
For example, in order to resolve a Schema that is or contains refs, it needs access to the registry.
Actually, every place in the code when a Ref schema is created, we also have access to the referenced schema object itself, so
a way to circumvent this would be to have a Schema that is a ref also hold either a pointer to the resolved ref or a function that returns the resolved schema.

For example:

type SchemaRef struct {
	Path   string  `yaml:"$ref,omitempty"`
	Schema *Schema `yaml:"-"`
}

type Schema struct {
	// ...
	Ref SchemaRef `yaml:",inline"`
	// ...
}

This would mean it is no longer necessary to pass the Registry object to the Validate function.
In my opinion, that could simplify the schema+registry internals ever so slightly, but I'm perfectly happy with what's here! Thanks again.

[danielgtaylor]

Thanks, that's an interesting approach to references and I like that schemas would contain all the information you need for e.g. validation. I'm going to merge this PR and will take a look at implementing your idea to see how it changes the code & interface because I can't visualize exactly how it may improve things without diving into the actual changes.