refactor(storage): client is templated on stubs

[dbolduc]

Part of the work for #2041

Template the client::Storage on the stub::Storage implementing it. I use S for Stub, and because no other generics used S. That kept the diff in check.

The default template is a transport::Storage in the client and the builders. This means code like: let client: Storage = Storage::builder().build().await keeps working as before.

The builder() method is only available on the "real" client. It only ever gives you a real client.

We need to make the default template type public, although it does not need to be name-able by applications.

The next PR will add the client::Storage::from_stub() method and make sure all types like RequestOptions are name-able outside of the crate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.41%. Comparing base (0492eb8) to head (5b817d9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3167      +/-   ##
==========================================
+ Coverage   96.39%   96.41%   +0.02%     
==========================================
  Files         110      110              
  Lines        4433     4433              
==========================================
+ Hits         4273     4274       +1     
+ Misses        160      159       -1     

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