internal/httpsfv: add parsing functionality for types defined in RFC 8941

This change introduces parsing functions for all item types defined in
RFC 8941, namely: integers, decimals, strings, tokens, byte sequences,
and booleans.

At this point, internal/httpsfv should be usable for parsing any RFC
8941-compliant HTTP Structured Field Values.

In a future CL, we will add support for parsing display strings and
dates, so that this package fully supports RFC 9651.

For golang/go#75500

Change-Id: Ib8ad2caa5f6ea4285d00506faa4b8127c2cc9419
Reviewed-on: https://go-review.googlesource.com/c/net/+/708435
Auto-Submit: Nicholas Husin <nsh@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>