floss: topshim: Add utility for easier sharing types with cxx bridge

Add gen_cxx_extern_trivial macro which generates the type info for cxx
bridge.

Use `namespace` attribute so we don't need to re-define the types in
type_alias.h

Bug: 343675216
Tag: #floss
Test: mmm packages/modules/Bluetooth
Flag: EXEMPT, Floss-only changes
Change-Id: Id177abdb4c4475ab3bdc03864d5f758c2d66150a