use generics instead as used within the original version
remove trait implementations for Box<dyn IO + Send> as no longer required

 replace direct field access with getter method that
 returns &Arc<impl TlsConnectorContext>,
 store concrete type in now private struct field

using impl Trait in return value keeps method signature
stable across features; allows for static dispatch

 adjust visibility for TlsConnectorCtx structs from pub(crate) to pub(super)