internal/httpsfv: add functionality to walk Parameters in HTTP SFV.

This change implements the minimum set of functionality within RFC 8491
that is needed in order for us to be able to extract information out of
Parameters type.

Rather than parsing the given Structured Field Values as usual, we
instead allow users to give us functions that will be invoked as we walk
through the SFV. This allows users to still extract information out of
SFV, without incurring significant memory allocation, especially when
the input is large.

If the current API & approach is good, we will proceed further by
implementing walk functionality for the rest of the types within RFC
8491: Dictionary, List, Item, and Inner List. After that, we will also
add support for Date and Display String to fully support RFC 9651.

For golang/go#75500

Change-Id: I838a7267a54fcd64b019be0ac10fe86b1e3e2c8b
Reviewed-on: https://go-review.googlesource.com/c/net/+/706755
Auto-Submit: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>