[Bitget Public] Add price related function in Bitget

[davidting0918]

• [x] 1. last_price
• [x] 2. index_price
• [x] 3. mark_price
• [x] 4. orderbook
• [x] 5. get_tickers
• [x] 6. get_ticker
• [x] 7. current_funding_rate
• [x] 8. history_funding_rate