List ResourceData Keys and set Values.

### SDK version

```
github.com/hashicorp/terraform-plugin-sdk/v2 v2.4.4
```

### Use-cases
I would like to copy all the values ("set" values) from a ResourceData to an internal struct.
At the moment you can call `Get` or `GetOk`. Instead, I would like a list of all these values.

### Attempted Solutions
```
One thought I had was to access the `schema` field in the ResourceData.
However, it's private and there is no getter.
```

### Proposal
Something like:
```
type ResourceDataAttr struct {
Key string
Value interface{}
}

func (*ResourceData) GetAll() []ResourceDataAttr {
...
}

or

func (*ResourceData) GetSchema() *schema.Schema {
...
}
```

### References

None at the moment.