chore: impl default for identity

[ra0x3]

Checklist

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Description

• I need to #[derive(Default)] for all types used by the indexer. Everything except Identity is working so far. I don't really care what it defaults to, just need a default for it.

[iqdecay]

@FuelLabs/sdk-rust I don't know about you all but I'm wary of giving default a meaning for an Identity. On the other hand, I totally understand the need for the derivability.
Also we are already pretty much doing this for AssetId so maybe it makes sense, just have to document that default in our context means null.

[segfault-magnet]

Also, we are already pretty much doing this for AssetId so maybe it makes sense, just have to document that default in our context means null.

Also, Address and ContractId are defaultable to 0.

I'd go ahead with the suggestion unless the rest of the team opposes it. Don't see somebody leaving the default value for the identity by mistake.

[hal3e]

LGTM