Define data model

[BytecodeBrewer]

What changed?

• Add internal market data models for data sources, instruments and price bars
• Represent FX rates as price bar close values
• Keep OHLCV fields optional for broader market data
• Add tests for data source, FX instrument, FX price bar and OHLCV-style market data

Related issue

Closes #42

Tests

• I ran pytest
• Existing tests pass
• I added or updated tests where necessary
• Not needed because this only affects documentation or repository setup

Checklist

• The change is focused
• No secrets or API keys are included
• Documentation was updated if needed
• CI is expected to pass

Notes

• methods for the classes will be included in later issues