CXX-2796 Restore external polyfill library and stdlib support (#1086)

* Add BSONCXX_USE_POLY_IMPLS to configuration

* Restore stdlib selection for make_unique, string_view, and optional

* CXX-641 fix usage of common flags in abidiff-test.sh

* Disable tests for external library polyfills

* Add changelog entry for ENABLE_BSONCXX_USE_POLY_IMPLS

* CXX-2797 Document ENABLE_BSONCXX_POLY_USE_IMPLS and deprecate external polyfill libraries

* CXX-2746 Ignore detail namespaces in abi-compliance-check-test.sh

* Add deprecation warnings for external polyfill library configurations

* Avoid including external symbols in v_noabi abidiff reports

* Ignore warnings for symbols unreferenced by debug info in abidiff

* Fix scoped_bson_t ambiguous overloads for views and values

* Add missing include in catch.hh

* Fix missing std::in_place equivalent for Boost before 1.63

* Improve quality of error messages for type trait assertions

* Relax trait equality assertions for stdlib implementations

* Declare BUILD_TESTING as a CMake option variable

* Fix CCACHE_BASEDIR for Windows distros

* Add missing macro guards for BSONCXX_POLY_USE_STD

* Revert reintroduction of documentation for experimental stdlib polyfill

* Move common polyfill selection documentation into separate page