Add validation mechanisms for writing JSON. Validate against a specification.

[stephenberry]

Add validation mechanisms for writing JSON. Validate against a specification.

[stephenberry]

We should be able to support both compile time and runtime validation. Initially validation should be based off of JSON schema.

[stephenberry]

There are two approaches to runtime validation. We can perform validation on parse, or we can perform validation on C++ memory. The later will be much more performant and I think should be the first focus of Glaze. We will use the same glz::json_schema, but we can use these structs at runtime to validate the memory of the C++ objects.

my_struct obj{};
if (glz::schema_validate(obj)) {
   // true if the object matches the gz::json_schema
   // note that we don't use a buffer, instead we would first read in a JSON file
   // and then we would validate
}