double_key! macro requires serde feature in downstream crates

[netrome]

Problem:

During the 1.85.0 bump, I got this error when building fuel-core:

error: unexpected `cfg` condition value: `serde`
  --> crates/fuel-core/src/graphql_api/storage/balances.rs:26:1
   |
26 | double_key!(CoinBalancesKey, Address, address, AssetId, asset_id);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: expected values for `feature` are: `backup`, `default`, `fault-proving`, `fuel-core-parallel-executor`, `p2p`, `parallel-executor`, `relayer`, `rocksdb`, `rocksdb-production`, `shared-sequencer`, `smt`, `test-helpers`, and `wasm-executor`
   = note: using a cfg inside a macro will use the cfgs from the destination crate and not the ones from the defining crate
   = help: try referring to `double_key` crate for guidance on how handle this unexpected cfg
   = help: the macro `double_key` may come from an old version of the `fuel_vm` crate, try updating your dependency with `cargo update -p fuel_vm`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

The problem is that the double_key macro contains a #[cfg(feature = "serde")] attribute, requiring any downstream crate using it to have a serde feature.

See slack thread for more context and solution proposals.

Definition of Done

The macro has been reworked so that we don't require serde for downstream crates.