Remove `serde` feature from the `fuel-tx` crate

[xgreenx]

Closes https://github.com/FuelLabs/fuel-vm/issues/832

After we introduced the Upgrade transaction and started to use postcard to encode the consensus parameters, we basically made the serde feature required.

Part of our codebase already derived serde::Seriliaze and serde::Deserialize regardless of the serde feature. This PR removes the serde feature and enables it for the fuel-tx crate by default under the alloc feature.

Checklist

• [x] Breaking changes are clearly marked as such in the PR description and changelog

Before requesting review

• [x] I have reviewed the code myself