Make base API URL configurable via env var and kwarg

[CollierKing]

Summary

• Connection and AsyncConnection now accept a base_url kwarg to override the hard-coded Cloudflare endpoint
• Falls back to the CF_D1_BASE_URL environment variable, then the default Cloudflare URL

Usage

# Recommended: env var (affects all connections in the process)
CF_D1_BASE_URL=http://localhost:8787

# Recommended: connect_args (clean, no URL encoding needed)
create_engine(
    "cloudflare_d1://acct:token@db",
    connect_args={"base_url": "http://localhost:8787"},
)

# Also works: direct kwarg on Connection
Connection(..., base_url="http://localhost:8787")

# Also works: URL query param (requires percent-encoding)
create_engine("cloudflare_d1://acct:token@db?base_url=http%3A%2F%2Flocalhost%3A8787")

Closes #22

Test plan

• 4 new unit tests in tests/unit/test_dialect.py covering kwarg, env var, default fallback, and query-param passthrough
• 5 new integration tests in tests/integration/test_base_url_integration.py — spin up a local HTTP server mimicking the D1 /raw response format and verify all override mechanisms route requests there (no Cloudflare credentials required)
• make test passes (20 unit tests)
• uv run pytest tests/integration/test_base_url_integration.py -v passes (5 integration tests)