Let a library provide mbedTLS for static builds #161

A library that bundles mbedTLS, such as Lime 8.4.0 (mbedTLS 3.6 for curl), puts a second copy into a static app next to libhxcpp's. In static builds the library should be able to offer its copy, so sys.ssl is built on it and the app links one mbedTLS.

Design: the library sets the define hxcpp_mbedtls_provider to a provider file. When the build is static and the provider declares mbedTLS 3, libhxcpp compiles SSL.cpp with the provider's flags, leaves its own mbedtls group out, and skips its threading setup. Upstream hxcpp ignores the define. Spec in docs/MbedTLSProvider.md.