Make CSV importer configurable via ConfigArgParse

[MrlixiangWE]

Problem

CSV is the only importer whose delimiter and quote character cannot be set through ConfigArgParse. They can currently only be configured per test in YAML.

Before / after

Before: CSV delimiter and quote character can only be configured through per-test csv_options in YAML.

After: they can also be set through --csv-delimiter and --csv-quote-char, the matching environment variables, or a csv section in the config file. Explicit global values override per-test options; otherwise existing behavior is unchanged.

Verification

uv run pytest -q --ignore=tests/graphite_e2e_test.py --ignore=tests/postgres_e2e_test.py

uv run tox -e lint

uv build --no-create-gitignore

Closes #119