Fix BtcConverter by migrating from decommissioned CoinDesk API to Coinbase

[LIGHTUNCHARGED]

The CoinDesk Bitcoin Price Index (BPI) API domain (api.coindesk.com) has been completely decommissioned. As a result, every method inside the BtcConverter class currently fails, throwing urllib3.exceptions.NameResolutionError / DNS resolution failures.
This PR restores full functionality to BtcConverter by replacing the dead CoinDesk endpoints in forex_python/bitcoin.py with Coinbase's stable, public, and keyless API endpoints:

Real-time spot price endpoint: /v2/prices/BTC-{currency}/spot

Historical price endpoint (by date): /v2/prices/BTC-{currency}/spot?date={YYYY-MM-DD}
All internal class methods, historical queries, and conversions (supporting both float and Decimal response formats) have been thoroughly updated and are now fully functional.

Changes Checklist
[x] Swapped dead api.coindesk.com URLs with active api.coinbase.com endpoints.

[x] Parsed the updated JSON response schema structure from the Coinbase API.

[x] Verified compatibility with both standard float and exact Decimal data returns.

[x] Tested all historical date parsing methods locally.

You can verify the fix by running the following end-to-end test script against the modified branch. It exercises every core method available in the BtcConverter class:

from datetime import datetime, timedelta
from forex_python.bitcoin import BtcConverter

b = BtcConverter()

print("--- Testing Live & Historical Price Fetching ---")
# Test get_latest_price
print("get_latest_price (EUR):", b.get_latest_price('EUR'))

# Test get_previous_price
date_obj = datetime.now() - timedelta(days=5)
print("get_previous_price (EUR, 5 days ago):", b.get_previous_price('EUR', date_obj))

# Test get_previous_price_list
start_date = datetime.now() - timedelta(days=3)
end_date = datetime.now() - timedelta(days=1)
print("get_previous_price_list (EUR):", b.get_previous_price_list('EUR', start_date, end_date))

print("\n--- Testing Currency Conversion Calculations ---")
# Test convert_to_btc
print("convert_to_btc (1000 EUR):", b.convert_to_btc(1000, 'EUR'))

# Test convert_btc_to_cur
print("convert_btc_to_cur (0.02 BTC to EUR):", b.convert_btc_to_cur(0.02, 'EUR'))

# Test convert_to_btc_on
print("convert_to_btc_on (1000 EUR, 5 days ago):", b.convert_to_btc_on(1000, 'EUR', date_obj))

# Test convert_btc_to_cur_on
print("convert_btc_to_cur_on (0.02 BTC to EUR, 5 days ago):", b.convert_btc_to_cur_on(0.02, 'EUR', date_obj))