feat!: require Backend to implement Default trait

[TomAFrench]

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Description

In our new merged acvm-backend-barretenberg crate, we're implementing the Backend trait directly on the Barretenberg struct rather than having a separate Plonk unit struct which then constructs Barretenberg when needed.

https://github.com/noir-lang/aztec_backend/blob/9fed6e52778d993d7742c46bbeec66087958c075/acvm_backend_barretenberg/src/lib.rs#L21-L26

This means that when we're actually using the backend in Nargo we need to construct the struct, i.e we need to call let backend = Barretenberg::default() rather than just being able to do let backend = Plonk.

Example: noir-lang/noir@ad822e6

We then want to enforce that this method exists on all backends so it must be included in the trait definition.

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Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
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• I have ensured all changes are covered in the description.

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