test: more local reset counts

The `Connection` struct contains a `tracing::Span` tracking its
lifetime. This can be useful. The problem is that this span has as its
parent the span within which the connection is created, meaning that it
keeps that span alive (not closed) while it is alive. In practice that
means, for example, instrumented functions that create connections and
store those connections for reuse will not have their spans closed and
exported when they end.

This commit sets the `Connection`'s span's parent to `None` and gives it
a "follows-from" relation to the span in which it was created instead.
This shows the causal relationship between them, without tying the
creating span's lifetime to the connection's lifetime.

We did not used to include padding when returning flow control. This now automatically releases any padded length if a DATA frame with padding is received.

Closes #867

Add support for HTTP/2 informational responses (1xx status codes) including
103 Early Hints. This enables servers to send preliminary headers before
the final response, improving client performance through early resource
discovery and connection establishment.

Changes include:
- extend client and server APIs to handle interim informational responses
- update stream state management for 1xx responses
- add test for interim informational response scenarios

This implementation follows RFC 7540 and RFC 8297 specifications for
HTTP/2 informational responses handling.

Co-authored-by: Apu Islam <apuislam@amazon.com>

`Prioritize::send_data` has a check to prevent assigning capacity to streams that are not yet open. Assigning flow control window to pending streams could starve already open streams.

This change adds a similar check to `Prioritize::reserve_capacity`.

Test `capacity_not_assigned_to_unopened_streams` in `flow_control.rs` demonstrates the fix.

A number of other tests must be changed because they were assuming that pending streams immediately received connection capacity.

This may be related to #853.

Co-authored-by: Sean McArthur <sean@seanmonstar.com>