feat(openapi3gen): Customize json.RawMessage

[kyleconroy]

Using a SchemaCustomizer, I can't configure schemas for json.RawMessage fields. If I have the following struct

type Foo struct {
    Bar string `json:"bar"`
    Output json.RawMessage `json:"output"`
}

The schema customization function will only be called for the Bar field, not the Output field.

[kyleconroy]

@fenollp please let me know if this is something you'd consider merging. Thanks!

[fenollp]

Please add tests.

[kyleconroy]

Tests added