Implement spender approvals and transfers

[tgntr]

Implements spender approval primitives for the asset pallet and adds a transfer_funds extrinsic to the settlement pallet for spending approved allowances and same-identity direct transfers.

changelog

new features

• Added spender allowance system — owners can approve other accounts to transfer fungible assets on their behalf
• Added transfer_funds to the settlement pallet — supports same-identity direct transfers and spender-approved transfers without going through the settlement engine
• Same-identity transfers for transfer_asset
• Added transfer_nft to the NFT pallet — transfer NFTs between accounts/portfolios with same routing logic as transfer_funds

modified external API

• transfer_asset now uses ensure_origin_call_permissions and routes same-DID transfers directly instead of always creating a settlement instruction

new external API

• Added approve extrinsic to the asset pallet — set, replace, or revoke a spender allowance for a given asset
• Added transfer_funds extrinsic to the settlement pallet — transfer assets with optional spender approval, defaults source to caller's account when from is None
• Added allowance(owner, spender, asset_id) Runtime API — query remaining spender allowance without a transaction
• Added transfer_nft extrinsic to the NFT pallet — transfer NFTs with same-DID direct / cross-DID settlement routing

new events

• Added Approval { owner, spender, asset_id, amount } event — emitted on approve calls, not on spend

other

• Added Allowances storage map to the asset pallet — maps (owner, spender, asset_id) to approved balance
• Added InsufficientAllowance error to the asset pallet
• Added SenderSameAsReceiver and AllowancesNotSupportedForNFTs errors to the settlement pallet
• Added spend_allowance public helper to the asset pallet — used by the settlement pallet to check and decrement allowances
• Infinite allowance (Balance::MAX) is never decremented on spend; allowance depleted to zero removes the storage entry
• Added tests for approve, allowance Runtime API, and transfer_funds covering spender mode, same-identity transfers, cross-identity settlement, portfolio custody, NFT transfers, atomicity, and edge cases
• Added PortfolioFnTrait / PortfolioFnConfig
• Added transfer_funds / transfer_funds_weight to SettlementFnTrait — exposes routing logic for cross-pallet callers
• simplified_nft_transfer now accepts Option<InstructionId> (was required)
• Removed dead code: transfer_asset_base_weight benchmark, transfer_asset_and_try_execute, transfer_nft_and_try_execute

[Neopallium]

Overview of changes for this PR:

1. Update the benchmarks/weights for NFTs.
2. Nft.transfer_nft use AccountId instead of AssetHolder for from/to.

For follow-up PR:

1. Use the fund, from, to parameters to select a more optimal worse-case weight (to lower the estimated tx fee).
2. Don't include compliance/transfer rule and execution cost in base weights, so that the meter can be used for calculating a better actual fee.

[Neopallium]

Right now the is_fungible flag is only set to true, since this method is only called for the fungible case. If we decide to add a Nft.receiver_affirm_nft_transfer extrinsic, then we will need to have the AssetCount include the correct number of NFTs.

For this PR, this is fine as is.