values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

ARR,
    OBJ,
    NONE, 
};
struct RPCArg {
    enum class Type {
        OBJ,
        ARR,
        STR,
        NUM,
        BOOL,
        OBJ_USER_KEYS, 
        AMOUNT,        
        STR_HEX,       
        RANGE,         
    };
    enum class Optional {