[RPC] rref.get_type() should have default return type of future

[rohan-varma]

🚀 Feature

#50977 is adding an option to make rref.get_type() not block. Per @mrshenli 's comments on that PR, it would be better if the getRRefType and _rref_typeof_on_owner return a future by default, so this function always returns a future. Then we can chain additional logic as a callback.

Open question: If we make this API public, should it by only return a future, or should we have an option to also block and return an actual type (as is the current behavior?)

cc @pietern @mrshenli @pritamdamania87 @zhaojuanmao @satgera @gqchen @aazzolini @rohan-varma @jjlilley @osalpekar @jiayisuse @agolynski @SciPioneer @H-Huang @mrzzd @cbalioglu