Update hardcoded VNXAU asset

[typicalHuman]

Description

Removed VNXAU hardcoded address and added logic to get it from config. Also updated the handlebars template for assets to just retrieve VNXAU_ADDRESS without looking in the array of fxAggregators every time. Added XAU/USD aggregator to the mainnet chain.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency changes
• Code refactor / cleanup
• Documentation or wording changes
• Other

How should this be tested?

yarn codegen and then yarn generate-assets, it should update helpers/assets.ts file with the new VNXAU_ADDRESS constant.

Checklist:

• I have performed a self-review of my own code
• I have requested at least 1 review (If the PR is significant enough, use best judgement here)
• I have commented my code where relevant, particularly in hard-to-understand areas

Merges to dev

• I have checked that the graft base is not a pruned deployment

[mendesfabio]

hey @typicalHuman - thanks for the contribution! we'll review your PR over this week -- we want to first promote the changes currently on dev to master before adding new stuff