rpc: Accept scientific notation for monetary amounts in JSON

Add a function `ParseFixedPoint` that parses numbers according
to the JSON number specification and returns a 64-bit integer.

Then this in `AmountFromValue`, rather than `ParseMoney`.

Also add lots of tests (thanks to @jonasschnelli for some of them).

Fixes issue #6297.

src/utilstrencodings.h

@@ -109,4 +109,11 @@ bool TimingResistantEqual(const T& a, const T& b)
     return accumulator == 0;
 }
 
+/** Parse number as fixed point according to JSON number syntax.
+ * See http://json.org/number.gif
+ * @returns true on success, false on error.
+ * @note The result must be in the range (-10^18,10^18), otherwise an overflow error will trigger.
+ */
+bool ParseFixedPoint(const std::string &val, int decimals, int64_t *amount_out);
+
 #endif // BITCOIN_UTILSTRENCODINGS_H