Remove serde feature from the fuel-tx crate

[xgreenx]

Closes #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

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

Before requesting review

• I have reviewed the code myself

[rymnc]

LGTM, curious if this was tested with the network upgrader which we ran into issues with serde? :)

[xgreenx]

No, I haven't tested it, but since we added assert into network ugprader itself, we will see if something is wrong=)