Add datasource BACKFILL skip support

[gnzjgo]

Closes #16.

Summary

• Adds backfill="skip" to DatasourceOptions / define_datasource.
• Emits BACKFILL skip in generated .datasource files.
• Parses legacy .datasource BACKFILL skip during migration and emits the equivalent Python option.
• Rejects unsupported backfill values so the SDK doesn't silently generate invalid directives.

Reproducibility

Before this change, the following datasource definition accepted the extra option but generated no BACKFILL directive:

from tinybird_sdk import define_datasource, t
from tinybird_sdk.generator.datasource import generate_datasource

daily_page_visits = define_datasource("daily_page_visits", {
    "schema": {
        "date": t.date(),
        "page_url": t.string(),
        "visits": t.uint64(),
    },
    "backfill": "skip",
})

print(generate_datasource(daily_page_visits).content)

After this change, the generated output includes:

ENGINE "MergeTree"
BACKFILL skip

I also added migration coverage for parsing a legacy .datasource containing BACKFILL skip and emitting Python with 'backfill': "skip".

Validation

• uv run --frozen pytest → 105 passed
• Manual repro script above with uv run python confirmed generated output contains BACKFILL skip.