internal/httpsfv: add support for consuming Display String and Date type

This CL adds consumeDisplayString() and consumeDate() function, meaning
that we can now consume all types that are defined within RFC 9651. In
future CL, we will add the corresponding parsing function for all the
types, so callers of this package will not have to implement their own
parsing / formatting.

For golang/go#75500

Change-Id: I90aa132d3ab1385b310d821997da13a095cd71bc
Reviewed-on: https://go-review.googlesource.com/c/net/+/708015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>