internal/httpsfv: add support for Bare Inner List and Item type.

This change implements the consume and Parse functions for both the Item
and Bare Inner List type. This is part of a chain of changes that is needed
in order for us to fully support HTTP Structured Field Values parsing as
defined in RFC 9651.

In future changes, we will utilize the support for Bare Inner List and Item
that is added here to support more complex types, namely Dictionary and
List.

Note that Bare Inner List is something we define on our own. We define a
Bare Inner List as an Inner List without the top-most parameter meant
for the Inner List. For example, the Inner List `(a;b c;d);e` would
translate to the Bare Inner List `(a;b c;d)`. We have done this because
the parameter of an Inner List will be exposed to the user via
ParseDictionary() or ParseList() too. By implementing Bare Inner List,
we can avoid having two ways of accessing the Inner List parameter, and
incurring the cost of a more complex implementation for Inner List and
other types that utilize Inner List (e.g. if we have consumeInnerList,
ParseDictionary will have to use consumeInnerList and backtrack the
consumption to separate out the InnerList parameter).

For go/golang#75500

Change-Id: I9b418d10b5755195d1cc3ff5f7ea211423bc4b48
Reviewed-on: https://go-review.googlesource.com/c/net/+/707099
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>