Add synthetic OHLCV data generator for testing (closes #70)

[Dhiyaahaq33]

Summary

Implements the suggestion in #70: a synthetic price series generator to use as a helper/environment data source for quickly testing agents/environments, or for pre-training on idealized patterns before real market data.

• Adds meta/data_processors/synthetic.py with generate_synthetic_data() (and the underlying SyntheticDataGenerator class), producing OHLCV data in FinRL-Meta's standard schema (tic, time, open, high, low, close, adjusted_close, volume).
• Supports three patterns as requested in the issue: sine (sine wave), trend (linear drift), and random_walk (Gaussian random walk), each with configurable noise (noise_std), amplitude/step size, and a seed for reproducibility.
• No network access or API keys required — fully offline/deterministic given a seed.
• Adds unit_tests/test_synthetic.py (8 tests, all passing locally): schema validation, multi-ticker independence, seed reproducibility, trend direction, and error handling for unknown patterns / unsupported time intervals.
• Adds a short "Synthetic data for testing" section to the README with a usage example, right after the Supported Data Sources table.

Kept this as a standalone, self-contained module rather than wiring it into the DataSource enum / _Base download machinery, to keep the change small and non-invasive — happy to extend it to full DataProcessor integration in a follow-up if maintainers prefer that shape.

Test plan

• python -m pytest unit_tests/test_synthetic.py -v — 8/8 passing
• Manually verified generated DataFrame shape/columns and that close stays positive across all three patterns
• Confirmed issue [Suggestion] Synthetic series for first "games" and / or testing #70 is still open with no other PR addressing it

Closes #70

[Dhiyaahaq33]

Note on the failing `pre-commit.ci` check: it isn't related to the files in this PR. Looking at the run log, the failures are all in pre-existing files unrelated to this change — ~29 files with a latent reorder-python-imports/black disagreement over blank lines after module docstrings, plus a real syntax error in meta/env_stock_trading/env_mintrades_stocktrading.py (Cannot parse: 44:8) that breaks black repo-wide. Neither meta/data_processors/synthetic.py nor unit_tests/test_synthetic.py appear in the reformatted/failed file list — this PR's own files pass all hooks cleanly when run in isolation. Happy to help clean up that pre-existing debt in a separate PR if useful, but wanted to flag it's out of scope here.