File logging support via --log-file option

[joseluisq]

Description

This PR adds support for File Logging to write log records to a file on disk, in addition to stderr. It is useful for production deployments where stderr is captured by a service manager, but a durable on-disk copy is also required.

--log-file <PATH> (SERVER_LOG_FILE): Filesystem path to stream log records to in addition to stderr. Missing parent directories are created on startup. The file is opened in append mode.

The file uses the format selected by --log-format (json by default) and the level selected by --log-level.
ANSI escape codes are always disabled for file output regardless of --log-with-ansi, so the file stays parsable.

Example

static-web-server \
    --port 8787 \
    --root ./my-public-dir \
    --log-level info \
    --log-format json \
    --log-file /var/log/sws/server.log

Note that SWS does not rotate the file itself, so an external tool such as logrotate could be used.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
src/settings/mod.rs	64% smaller
src/logger.rs	11% smaller
Cargo.lock	Unsupported file format
Cargo.toml	Unsupported file format
docs/content/configuration/command-line-arguments.md	Unsupported file format
docs/content/configuration/environment-variables.md	Unsupported file format
docs/content/features/logging.md	Unsupported file format
src/settings/cli.rs	0% smaller
src/settings/file.rs	0% smaller
tests/file_logging.rs	0% smaller