Converting to/from v1/v2 extension descriptor types is a common operation
for v1 and v2 interoperability. Optimize these operations with a cache.
Change-Id: I5feca810f60376847c791654982acd3b6a37a5db
Reviewed-on: https://go-review.googlesource.com/c/152542
Reviewed-by: Herbie Ong <herbie@google.com>
