Allow setting s3_url_style via environment variable

The other S3 settings can be set via `AWS_*` / `DUCKDB_S3_*` environment variables, but `s3_url_style` cannot. S3-compatible endpoints often need `path` style, so it currently requires `SET s3_url_style = 'path'` on every connection.

PR: #437

Start with PR #437 and compare its handling of the existing AWS_* and DUCKDB_S3_* environment variables with the s3_url_style setting described here. Done means s3_url_style can be configured through an environment variable and no longer requires SET on every connection; verify the behavior with the relevant existing tests.

Written by the indexing model from the issue text.