feat: add per-connection attachment storage to HttpConnectionInternal

[Harshal96]

Motivation:

vert-x3/vertx-web#2910 needs per-connection state (announced Alt-Svc origins) scoped to the connection's lifecycle. The current workaround in vertx-web is a WeakHashMap<HttpConnection, T>, which can't reliably clean up stale connections (see discussion in that PR).

Changes

New HttpConnectionInternal interface in io.vertx.core.internal.http, following the existing HttpServerRequestInternal / HttpClientRequestInternal pattern:

<T> T get(Object key);
void set(Object key, Object value);

Backed by a lazily-created map stored directly on the connection object (not an external map keyed by connection), so entries are naturally scoped to the connection's own lifecycle — no leak risk, no weak references needed.

Wired into all four connection implementations:
ConnectionBase (shared by Http1Connection, Http2ConnectionImpl, Http2MultiplexConnection), cleared in handleClosed(). Http3Connection, which doesn't extend ConnectionBase, gets its own field and clears it in its own handleClosed().

Fixes #6234

[Harshal96]

@vietj something like this?

[vietj]

ping @Harshal96

[vietj]

Can you instead of using a map with synchronization around instead use a ConcurrentHashMap implementation ?

[Harshal96]

@vietj done