extensions: support providing extensions in ogen.Spec, that can be marshalled to JSON

[lrstanley]

Description

I have a project that uses entgo and its associated extension entoas to generate a schema specification using ogen. It does this by using the ent graph, which has all type information and annotations to control what to add and exclude from the specification. This works relatively well today, however, it currently isn't possible to add OpenAPI extensions (x-<etc>) to the spec, and have it be marshalled to JSON.

This looks to be due to the json:"-" struct tags on all of the related extension fields, for example:

ogen/spec.go

Lines 83 to 84 in b41f78c

	// Specification extensions.
	Extensions Extensions `json:"-" yaml:",inline"`

I suspect this is due to encoding/json not supporting inlining for non-embedded fields, however, I am wondering if there is a way around this (maybe marshal to yaml, then to json in all MarshalJSON() methods, if the yaml encoder supports inlining? Maybe go-faster/jx could be used (or may be overkill?)

References

n/a