Propose and approve a spend of treasury funds.

## Dispatch Origin

Must be [`Config::SpendOrigin`] with the `Success` value being at least
`amount` of `asset_kind` in the native asset. The amount of `asset_kind` is converted
for assertion using the [`Config::BalanceConverter`].

## Details

Create an approved spend for transferring a specific `amount` of `asset_kind` to a
designated beneficiary. The spend must be claimed using the `payout` dispatchable within
the [`Config::PayoutPeriod`].

### Parameters
- `asset_kind`: An indicator of the specific asset class to be spent.
- `amount`: The amount to be transferred from the treasury to the `beneficiary`.
- `beneficiary`: The beneficiary of the spend.
- `valid_from`: The block number from which the spend can be claimed. It can refer to
  the past if the resulting spend has not yet expired according to the
  [`Config::PayoutPeriod`]. If `None`, the spend can be claimed immediately after
  approval.

## Events

Emits [`Event::AssetSpendApproved`] if successful.

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#236 kjjh

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