[btls] Convert BTLS icalls to pinvokes by invoking them using [DllImp… (#3799)

[btls] Convert BTLS icalls to pinvokes by invoking them using [DllImp… (#3799)

* [btls] Convert BTLS icalls to pinvokes by invoking them using [DllImport("__Internal")], which will make it easier to redirect them to a separate dylib in the future.

* [btls] Add a --enable-dynamic-btls configure flag to enable compiling btls into a separate shared library instead of embedding it into the runtime.