feat: put wasm-bindgen-futures dep behind the wasm-bindgen feature flag

[leruaa]

Closes #792

[leruaa]

fyi, wasm-bindgen is brought by a few other paths:

cargo tree --target wasm32-unknown-unknown

alloy v0.1.0 (/workspaces/crypto/alloy/crates/alloy)
├── alloy-core v0.7.2
│   └── alloy-primitives v0.7.2
│       ├── getrandom v0.2.12
│       │   ├── cfg-if v1.0.0
│       │   ├── js-sys v0.3.69
│       │   │   └── wasm-bindgen v0.2.92
│       │   └── wasm-bindgen v0.2.92 (*)
└── reqwest v0.12.1
    ├── wasm-bindgen v0.2.92 (*)
    ├── wasm-bindgen-futures v0.4.42
    │   └── wasm-bindgen v0.2.92 (*)
    └── web-sys v0.3.69
        └── wasm-bindgen v0.2.92 (*)

alloy-contract v0.1.0 (/workspaces/crypto/alloy/crates/contract)
├── alloy-provider v0.1.0 (/workspaces/crypto/alloy/crates/provider)
│   ├── alloy-rpc-client v0.1.0 (/workspaces/crypto/alloy/crates/rpc-client)
│   │   ├── alloy-transport-ws v0.1.0 (/workspaces/crypto/alloy/crates/transport-ws)
│   │   │   └── ws_stream_wasm v0.7.4
│   │   │       ├── wasm-bindgen v0.2.92 (*)
│   │   │       ├── wasm-bindgen-futures v0.4.42 (*)

alloy-signer-ledger v0.1.0 (/workspaces/crypto/alloy/crates/signer-ledger)
├── coins-ledger v0.10.1
│   ├── wasm-bindgen v0.2.92 (*)
│   └── wasm-bindgen-futures v0.4.42 (*)

And also alloy-signer-gcp via gcloud-sdk -> reqwest