gddo-server/dynconfig: add package

Copied from https://go.googlesource.com/pkgsite/+/cb9361e611fe25baabab637bc260f48b345bd1da/internal/config/dynconfig
with minor modification to remove reference to derrors package.

Change-Id: Ib006861369613183ac29eb1456582e34dbd83b22
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285834
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>