common/strings: Support classes with operator<<(std::ostream&, const T&)

This allows to reuse all the existing string conversion logic for custom
classes for which we already have these defined:
  std::ostream& operator<<(std::ostream& os, const T& obj)

Bug: 150670922
Tag: #feature
Test: atest packages/modules/Bluetooth/system/gd/common/strings_test.cc
Sponsor: jpawlowski@
Change-Id: Ie31584231800573b5dbc59caba1cccc0c0787be2