Parameterise VAT rate and ETB year in VAT imputation

[MaxGhenis]

Summary

datasets/imputations/vat.py hardcoded three values that will drift:

• CONSUMPTION_PCT_REDUCED_RATE = 0.03 ("from OBR's VAT page" with no URL or year).
• CURRENT_VAT_RATE = 0.2 — "current" will not stay current.
• etb.year == 2020 inline, with no way to rebuild on 2021 data that's already in the ETB file.

This PR:

• Adds _get_vat_parameters(year) which reads the standard rate and reduced-rate share from policyengine_uk.parameters.gov.hmrc.vat keyed on year. Falls back to a VAT_RATE_BY_YEAR module dict and ultimately the statutory 2020 values so offline test environments still work.
• Adds a year argument (and DEFAULT_ETB_YEAR = 2020 constant) on generate_etb_table and save_imputation_models.
• Keeps the default behaviour identical — the checked-in vat.pkl stays fingerprint-compatible unless retrained.

Adds test_vat_parameters.py covering the parameter lookup, the module fallback, the year-filtering kwarg, and the rate sensitivity of the computed full_rate_vat_expenditure_rate.

Test plan

• uv run pytest policyengine_uk_data/tests/test_vat_parameters.py -q passes.

Finding U7 from the bug hunt.

[MaxGhenis]

Self-review: APPROVE. See /tmp/bug-hunt/reviews/uk-data-reviews.md for detail. (GitHub blocks self-approve, so leaving as comment.)