Introduce Fee Calculation for AccountDelete and AmmCreate to account for custom logic.

[sappenin]

#585 fixed fee calculation in EscrowFinish, but as part of that work it was noticed that there isn't any helpful function for computing fees for other transactions that don't use standard base fees.

For example, see AccountDelete and AmmCreate, and possibly multisig (though this is probably working correctly from a fee perspective).

See these links in the other SDKs:

• https://github.com/XRPLF/xrpl.js/blob/main/packages/xrpl/src/sugar/autofill.ts#L258
• https://github.com/XRPLF/xrpl.js/blob/main/packages/xrpl/src/sugar/getFeeXrp.ts