[Deprecation] deprecate treasury spend_local call and related items

[davidk-pt]

Resolves #5930

spend_local from treasury pallet and associated types are deprecated. spend_local was being used before with native currency in the treasury.

This PR provides a documentation on how to migrate to the spend call instead.

Migration

For users who were using only spend_local before

To replace spend_local functionality configure Paymaster pallet configuration to be PayFromAccount and configure AssetKind to be () and use spend call instead.
This way spend call will function as deprecated spend_local.

Example:

impl pallet_treasury::Config for Runtime {
    ..
    type AssetKind = ();
    type Paymaster = PayFromAccount<Self::Currency, TreasuryAccount>;
    // convert balance 1:1 ratio with native currency
    type BalanceConverter = UnityAssetBalanceConversion;
    ..
}

For users who were already using spend with all other assets, except the native asset

Use NativeOrWithId type for AssetKind and have a UnionOf for native and non-native assets, then use that with PayAssetFromAccount.

Example from kitchensink-runtime:

// Union of native currency and assets
pub type NativeAndAssets =
    UnionOf<Balances, Assets, NativeFromLeft, NativeOrWithId<u32>, AccountId>;

impl pallet_treasury::Config for Runtime {
    ..
    type AssetKind = NativeOrWithId<u32>;
    type Paymaster = PayAssetFromAccount<NativeAndAssets, TreasuryAccount>;
    type BalanceConverter = AssetRate;
    ..
}

// AssetRate pallet configuration
impl pallet_asset_rate::Config for Runtime {
    ..
    type Currency = Balances;
    type AssetKind = NativeOrWithId<u32>;
    ..
}