Adds a new Version type to pkgbits to represent the version of the
bitstream. Versions let readers and writers know when different data is
expected to be present or not in the bitstream. These different pieces
of data are called Fields, as an analogy with fields of a struct.
Fields can be added, removed or changed in a Version. Extends Encoder
and Decoder to report which version they are.

Updates #68778

Change-Id: Iaffa1828544fb4cbc47a905de853449bc8e5b91f
Reviewed-on: https://go-review.googlesource.com/c/go/+/605655


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>