internal/httpsfv: implement parsing support for Dictionary and List type.

This change implements the Parse functions for the Dictionary and List
type. At this point, we should be able to use internal/httpsfv package
to extract information from any HTTP SFV that follows RFC 8941.

In future changes, we will add additional types introduced in RFC 9651
to achieve feature parity with it. Additionally, we will add Parse
functions for all the HTTP SFV types, such that users of the package do
not need to do their own type assertions and conversions.

Note that the Dictionary and List type do not have a consume function.
This is because both types never appear as a child of other types,
meaning it is guaranteed to always consume its entire string input.

For go/golang#75500

Change-Id: I376dca274d920a4bea276ebb4d49a9cd768c79fe
Reviewed-on: https://go-review.googlesource.com/c/net/+/707100
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>