[Bug] processor_ccxt.py: Local timezone conversion causes index drift in CCXTEngineer.data_fetch

### Summary of the Defect

In `finrl/meta/data_processors/processor_ccxt.py`, `CCXTEngineer.data_fetch` converts exchange millisecond timestamps into DataFrame datetime indices using naive `datetime.fromtimestamp()`:

```python
# finrl/meta/data_processors/processor_ccxt.py lines 49-51
df["time"] = [
datetime.fromtimestamp(float(time) / 1000) for time in df["time"]
]
```

Because `datetime.fromtimestamp()` without an explicit timezone parameter defaults to the local OS timezone, executing this data processor on machines configured in non-UTC regions (e.g. UTC+5:30, UTC-5, UTC+8) produces local naive datetimes.

### Inconsistency Breakdown

1. **Exchange Contract Violation:** The CCXT exchange protocol returns standard Unix epoch millisecond timestamps in UTC.
2. **Query Window vs Index Mismatch:**
- Lines 66–67 use `calendar.timegm(start_dt.utctimetuple())` which treats the naive string input as UTC.
- Lines 70 and 75 generate window slices using `datetime.utcfromtimestamp()` (which is also deprecated in Python 3.12+).
- But lines 49–51 parse the fetched rows using local time.
3. **Downstream Quant / RL Impact:**
- **Multi-Pair Misalignment:** When aligning crypto pairs or combining CCXT data with other data sources (`YahooFinanceProcessor`, `AlpacaProcessor`), timestamps are offset by the local machine's UTC offset, producing corrupted cross-asset matrices.
- **Feature Drift:** Technical indicator calculations (RSI, MACD) risk time-shift errors when joined against UTC-indexed macro feeds.

### Minimal Standalone Reproduction (Zero Network Dependency)

```python
import os
import time
from datetime import datetime, timezone

# 1704067200000 ms is exactly 2024-01-01 00:00:00 UTC
exchange_ms = 1704067200000

# Current FinRL implementation:
current_parsed = datetime.fromtimestamp(float(exchange_ms) / 1000)

# Expected UTC implementation:
expected_utc = datetime.fromtimestamp(float(exchange_ms) / 1000, tz=timezone.utc)

print(f"Raw Exchange Timestamp: {exchange_ms}")
print(f"Current processor output (Local): {current_parsed}")
print(f"Expected canonical output (UTC) : {expected_utc}")

# On any non-UTC system, this assertion fails:
# assert current_parsed.hour == 0
```

### Proposed Fix

Replace naive conversions and deprecated `utcfromtimestamp` calls with `datetime.fromtimestamp(..., tz=timezone.utc)`:

```diff
--- a/finrl/meta/data_processors/processor_ccxt.py
+++ b/finrl/meta/data_processors/processor_ccxt.py
@@ -1,7 +1,7 @@
from __future__ import annotations

import calendar
-from datetime import datetime
+from datetime import datetime, timezone

import ccxt
import numpy as np
@@ -48,7 +48,7 @@ class CCXTEngineer:
df = pd.DataFrame(
ohlcv, columns=["time", "open", "high", "low", "close", "volume"]
)
- df["time"] = [
- datetime.fromtimestamp(float(time) / 1000) for time in df["time"]
+ df["time"] = [
+ datetime.fromtimestamp(float(time) / 1000, tz=timezone.utc) for time in df["time"]
]
df["open"] = df["open"].astype(np.float64)
@@ -69,12 +69,12 @@ class CCXTEngineer:
if period == "1m":
date_list = [
- datetime.utcfromtimestamp(float(time))
+ datetime.fromtimestamp(float(time), tz=timezone.utc)
for time in range(start_timestamp, end_timestamp, 60 * 720)
]
else:
date_list = [
- datetime.utcfromtimestamp(float(time))
+ datetime.fromtimestamp(float(time), tz=timezone.utc)
for time in range(start_timestamp, end_timestamp, 60 * 1440)
]
```

### Environment
- **OS:** Windows / Linux / macOS
- **Python:** 3.10 / 3.11 / 3.12+
- **FinRL:** `master`

Happy to open a PR with unit tests covering this fix if the maintainers agree with this direction.