Add per-connection attachment storage to HttpConnectionInternal

### Describe the feature

vert-x3/vertx-web#2910 needs per-connection state (announced Alt-Svc origins) that
must not leak after the connection closes. Current workaround lives entirely in
vertx-web: a `WeakHashMap` (see HttpConnectionLocal.java in that PR).
https://github.com/vert-x3/vertx-web/pull/2910#discussion_r3542072153 (comment by @vietj):
"actually I meant I would add a map like structure on vertx internal HTTP connection
object in vertx-core which would solve the issue more directly"

#### Proposal
Add attachment storage to the `HttpConnectionInternal` contract (new interface,
following the existing `HttpServerRequestInternal` / `HttpClientRequestInternal`
pattern in `io.vertx.core.internal.http`), scoped to the connection's lifecycle so
entries are cleared automatically on close:
```
T get(Object key);
void set(Object key, Object value);
```

Downstream modules (vertx-web, etc.) can then stash per-connection state via this
contract instead of each reinventing weak-map/close-handler bookkeeping.

### Contribution

Myself

Start with the HttpServerRequestInternal and HttpClientRequestInternal patterns in io.vertx.core.internal.http, then trace the HttpConnectionInternal close lifecycle. Review HttpConnectionLocal.java in the linked vertx-web work as the usage reference; done means downstream code can attach per-connection state and entries are cleared when the connection closes.

Written by the indexing model from the issue text.