Fix serialization of FeeSettings object

[mDuo13]

The FeeSettings ledger object type has some weird behavior when it's serialized to JSON (In ledger_entry or ledger_data responses, or in the metadata of a SetFee transaction):

• BaseFee is serialized as hex despite being an XRP amount (UInt64)
• ReferenceFeeUnits, ReserveBase, and ReserveIncrement are serialized as JSON numbers, not strings as XRP usually is. (This is because they are actually UInt32 types instead of the usual UInt64.)

It would be nice to change the binary format of the FeeSettings object to make it use the proper data type for XRP amounts in all its XRP fields, but that's harder to fix because it would require an amendment.

[mDuo13]

Note that with the given data structures the maximum theoretical values for fee settings are, shall we say, strange. The reserve settings, which would make sense to be much larger than the costs of sending a transaction, are more limited.

[intelliot]

Related: #4425

[intelliot]

@mDuo13 can you confirm this issue has been fixed as of what's in develop now?

[intelliot]

mDuo13 noted that this is probably fine. If anyone sees any potential issues, please comment here or create a new issue.