Add high-level methods for ledger

[LimpidCrypto]

• [x] get_fee
  1. Queries the Ledger for the current transaction fee.

2. The function calls the Fee request method using the WebsocketClient/AsyncWebsocketClient.

3. The user can decide what type of fee to get. "open" would return the open_ledger_fee field; "minimum" would return the minimum_fee field; "dynamic" would return a calculated fee based on the node's transaction queue size.

fields	type	explanation
client	WebsocketClient/AsyncWebsocketClient	A websocket client object.
max_fee	Option<u32>	The max fee the user want to spend as drops.
fee_type	Option<&str>	The type of fee to return. Must be one of: "open", "minimum", "dynamic"

Sources:

• https://github.com/XRPLF/xrpl-py/blob/master/xrpl/asyncio/ledger/main.py#L52

• [x] get_latest_validated_ledger_sequence
  1. Queries the Ledger for the sequence number of the last validated Ledger.

2. The function simply calls the Ledger request method and returns the results ledger_index field

3. The ledger_index field must be set to "validated".

fields	type	explanation
client	WebsocketClient/AsyncWebsocketClient	A websocket client object.

Sources:

• https://github.com/XRPLF/xrpl-py/blob/master/xrpl/asyncio/ledger/main.py#L12

• [ ] get_latest_open_ledger_sequence
  1. Queries the Ledger for the sequence number of the last open Ledger.

2. The function simply calls the Ledger request method and returns the results ledger_index field

3. The ledger_index field must be set to "open".

fields	type	explanation
client	WebsocketClient/AsyncWebsocketClient	A websocket client object.

Sources:

• https://github.com/XRPLF/xrpl-py/blob/master/xrpl/asyncio/ledger/main.py#L32