async-trait: make it an optional dependency

[bug-ops]

Problem

async-trait is declared as an unconditional dependency in crates/rmcp/Cargo.toml, but
every use of it sits behind non-default features:

• CredentialStore / StateStore (transport/auth.rs) — feature auth
• EventStore / SessionStore (session/store.rs) — feature transport-streamable-http-server-session

Default features (base64, macros, server) pull in neither, yet the proc-macro crate
still compiles for every consumer:

$ cargo tree -p rmcp -e normal --no-default-features --features base64,macros,server -i async-trait
async-trait (proc-macro)
└── rmcp

Proposal

# crates/rmcp/Cargo.toml
async-trait = { version = "0.1.89", optional = true }

[features]
auth = [..., "dep:async-trait"]
transport-streamable-http-server-session = [..., "dep:async-trait"]

Dev-dependency stays as-is (used directly by the event/session store integration tests).
Non-breaking, no public API change — trims the default build graph only.

[bug-ops]

Why not remove it entirely?

All 15 #[async_trait] sites are on traits consumed as Arc<dyn Trait>. Async fn in traits
(stable since 1.75, below our MSRV) doesn't apply — it isn't dyn-compatible, and no
async_fn_in_dyn_trait stabilization exists yet. The traits that don't need dyn dispatch
(Service, ServerHandler, ClientHandler, Transport) are already on hand-written RPITIT
with a MaybeSendFuture marker (service.rs:31-40) — that migration is done; this is what's
left over, by design.

Hand-boxing the remaining 4 traits (pattern already exists for Service/DynService,
service.rs:323-437) would add what the macro generates.