Add DatasetSchema and ColumnSchema core infrastructure

[GaganNarula]

Summary

• Adds esp_data/schema.py with DatasetSchema, ColumnSchema, and SchemaValidationError
• Adds get_dtype() method to DataBackend protocol and its pandas/polars implementations
• Adds optional schema class attribute and _validate_schema() method to base Dataset class
• Exports schema classes from esp_data.__init__

Part of

Stacked PR 1/4 for issue #227. Subsequent PRs add tests, per-dataset schemas, and description fields.

Test plan

• Unit tests will be added in the next PR in the stack
• Verify from esp_data import DatasetSchema, ColumnSchema works

[GaganNarula]

@mil-ad I think we could discuss this, and then with our answers it'll be easier to guide schema development ?