Add default date format option for file loads

[sawirricardo]

Summary

• accept WITH date format ... for CSV and fixed-width file loads
• apply the default format to target date/time columns while preserving per-field overrides
• document the option and update date-format parser/regression coverage

Fixes #1191.

Verification

• git diff --check
• docker run --rm -e USER=pgloader -v "$PWD/clojure":/work -w /work clojure:temurin-21-tools-deps clojure -M:test -m cognitect.test-runner -n pgloader.load-file.parser-test
• SBCL reader check for edited Lisp source/parser files
• Clojure reader check for edited Clojure parser files