chore: seal traits

[MujkicA]

Following the discussions around #1244 I figured we could still benefit from sealing some of our public traits. The benefit is that adding new methods to these traits will not cause a breaking change:

Transaction TransactionBuilder BuildableTransaction EstimablePredicates GasValidation

I've wondered wether Account is a trait where downstream implementations would make sense. I came up with a potential example of a joined account here. The current way to implement it is a bit clunky, especially because signing would require some kind of internal state to keep track of what accounts actually contributed to the input set. But if this is a a valid use case, we could consider improving the interface.

Checklist

• [ ] I have linked to any relevant issues.
• [ ] I have updated the documentation.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added necessary labels.
• [x] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [x] I have requested a review from the relevant team or maintainers.