[feature]: Add support of `propertyNames`.

Hi there,

I have the following types

**main.go**

```go
var (
_ huma.SchemaTransformer = Key("")
_ huma.SchemaTransformer = Value("")
)

type (
Key string
Value string
)

func (k Key) TransformSchema(r huma.Registry, s *huma.Schema) *huma.Schema {
min, max := 1, 10
s.Description = "A key for a map."
s.MinLength = &min
s.MaxLength = &max
return s
}

func (v Value) TransformSchema(r huma.Registry, s *huma.Schema) *huma.Schema {
min, max := 1, 32
s.Description = "A value for a map."
s.MinLength = &min
s.MaxLength = &max

return s
}

type Input struct {
Body struct {
Values map[Key]Value `json:"values" `
}
}

type Output struct {
Body struct {
Message string `json:"message"`
}
}

func addRoutes(api huma.API) {
huma.Post(api, "/input", func(ctx context.Context, input *Input) (*Output, error) {
resp := &Output{}
resp.Body.Message = "It works!"
return resp, nil
})
}
```

One parameter, `values,` is a map of `Key` and `Value`.
`Key` and `Value` have validation rules ( in `TransfromSchema` ).

**The issue.**

1) `Key` validation doesn't work.

**main_test.go**
```
func TestUserCreate(t *testing.T) {
_, api := humatest.New(t, huma.DefaultConfig("Test API", "1.0.0"))
addRoutes(api)

_ = api.Post("/input", map[string]any{"values": map[string]any{"": "value"}})
}
```

Temporary workaround: implementation of `huma.Resolver/huma.ResolverWithPath` interface.

2) Generated OAS doesn't have a `Key` schema.

```yaml
InputBody:
additionalProperties: false
properties:
$schema:
description: A URL to the JSON Schema for this object.
examples:
- https://example.com/schemas/InputBody.json
format: uri
readOnly: true
type: string
values:
additionalProperties:
description: A value for a map.
maxLength: 32
minLength: 1
type: string
type: object
required:
- values
type: object
```

`Value` type is represented by `additionalProperties`.

`Key` may be represented by `propertyNames` like

```yaml
...
values:
propertyNames:
type: string
maxLength: 10
minLength: 1
additionalProperties:
description: A value for a map.
maxLength: 32
minLength: 1
type: string
type: object
...
```

From what I've checked in the source code, `propertyNames` is not supported yet.

Could you please briefly describe what needs to be considered for the implementation in the source code ( if someone wants to contribute it )?

Thank you.