feat: make all instance attributes on HasRid subclasse private

[drake-nominal]

In a recent PR of mine, @stefanv had an excellent observation:

• Users will find it confusing how you cannot update one of our HasRid subclasses as such:

  ds = client.get_dataset("... rid ...")
  ds.name = "my cool new name"

• I have the additional observation that as our API changes, by preventing users from depending on our actual class attributes and instead exposing properties, we shield ourselves from having to make breaking changes as often. This can be seen with what happened with DataSource and the whole RID fiasco re: Channels.

Being the quick typer i am (aka regex) i have gone ahead and turned all API attributes into readonly properties.

Now, you might say, "but drake! our dataclasses are frozen, users should know they can't update these fields even in the first place!"

• maybe they can't do ds.properties = {"very cool property name": "kinda lame property value"}, but they sure can still do ds.properties["python is cool"] = "but rust is wack".
• See observation 2 above-- even if they couldn't modify mutable member variables, I claim we still don't want them accessing the member variables to begin with.
• You'd be surprised how many people (that aren't python library developers) aren't intimately familiar with dataclasses to even consider this.

[drake-nominal]

After discussion in slack, this was deemed not necessary