cleanup!: hide PageableResponse trait

[dbolduc]

Part of the work for #1609

Move PageableResponse to an internal module, and hide it from the docs.

When an RPC is paginated, we generate a paginator() method on the builder. Applications do not care that a message is a PageableResponse.

Note that in the future we will hide Paginator construction (currently, Paginator::new()) from applications as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.03%. Comparing base (78c6bca) to head (c9c2354).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1666   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files          47       47           
  Lines        1890     1890           
=======================================
  Hits         1815     1815           
  Misses         75       75           

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