USDx treasury spend proposal template

[Popoulosss]

Please check this issue #4224 first

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• Implement the quick start process for USDx treasury proposal
• Support switching the requested asset: USDt(default)/USDC. And need to show the corresponding treasury balance
• Add a prompt below the Benificiary input to inform users that need to fill in the AssetHub address
  • show tooltip, link to https://wiki.polkadot.network/docs/learn-assets
• Add a input Valid From which is need to be filled relay chain block number to the advanced settings, the default value is 0
  • show tooltip

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