Include setFee GraphQL mutation in updateAsset instead

[JoblersTune]

Prior to feat: liquidity webhooks #1782 there was an updateWithdrawlThreshold mutation for assets. But since liquidity updates were added as well now there's an updateAsset mutation. Since there is a general updateAsset mutation it would make sense to move the setFee mutation into the updateAsset mutation as well.

The createAsset mutation should also have an option to add fees immediately then as well.

[JoblersTune]

The decision is to leave setFee separate from creating and updating assets. Bringing fees into the asset creation and update functionality adds complexity. Assets and fees are stored in separate database tables, so it's logical to have separate mutations that respect the independence of these entities, as they can succeed or fail independently. This also simplifies error handling on both the frontend and backend as well as simplifying testing and debugging. Ultimately I think the code is cleaner if we keep the stricter separation of concerns.