server: ConnectionConfig — one per-connection settings value held by Server and BoundServer

[rpb-ant]

Implements #250 items 2–3, modelled on ClientConfig from #247. Most of the server.rs diff moves the long setter docs from BoundServer onto ConnectionConfig; git diff --color-moved=dimmed-zebra separates the moves from the edits.

Before this, nine per-connection fields were duplicated on Server and BoundServer, with four hand-synchronised default blocks and a nine-argument serve_with_listener. After, each builder holds one value, and the same value configures either:

let config = ConnectionConfig::new()
    .with_max_connection_age(Duration::from_secs(600))
    .with_http2_max_header_list_size(64 << 10);
Server::new(router).with_connection_config(config.clone()).serve(addr).await?;
// or: Server::bind(addr).await?.with_connection_config(config).serve(router).await?;

• Public ConnectionConfig, also at the crate root (#[non_exhaustive]; derives Clone, Debug, PartialEq, Eq), holds every setting the builders apply to a connection. The builders' existing with_* setters keep their signatures and edit that value; with_connection_config / connection_config() replace or return it whole.
• New setting with_http2_max_header_list_size(u32), the advertised SETTINGS_MAX_HEADER_LIST_SIZE (hyper's 16 KiB default is too small for some bearer tokens), with shorthands on Server and BoundServer. It panics on zero. A request whose header list reaches the limit gets 431. With h2 0.4.15 and later, a connection-closing GOAWAY follows a header list past four times the limit, or a header block spread over more CONTINUATION frames than h2 allows, which can happen well under four times the limit.
• The TLS settings move into a crate-private AcceptConfig, so the accept loop is serve_with_listener(listener, service, accept, config, shutdown) in every feature set.

[iainmcgin]

[claude code] Approving under the maintainers' agreement to review and merge each other's PRs. Rebased onto main after #306 merged; the added fix commits went through code, API, doc and deslop review.