Add option to log as JSON

[blevkivskyi-everstake]

Main points

To simplify log analysis across different monitoring stacks, please add an option to output logs in JSON format. While export JAVA_OPTS="-Dlog4j.configurationFile=<path_to_file>" allows for custom log configurations, it is a bit of an overhead for most users.

Proposed Changes

Add a CLI flag --log-format=json

[usmansaleem]

Web3Signer uses log4j library which allows users a lot of customization in terms of the logging pattern, where to write logs, log levels for various classes etc. Web3Signer bundles a default log4j configuration file that utilizes PatternLayout by default, which can be overridden by specifying JAVA_OPTS with Dlog4j.configurationFile=<path>.

The problem with specifying --log-format=json opens up issue of what kind of JSON format user is expecting. The latest release of Web3Signer now bundles an additional log4j library that allows production level JSON structured logging. See #1140 . This allows users to configure log4j to specify ECS, GCP and other JSON logging standard with flexibility to direct logging output to console or to single file or rolling files based on their requirements.

[blevkivskyi-everstake]

In automated or immutable infrastructure, injecting a custom xml file is more cumbersome than setting an environment variable or a CLI flag.

Since Web3Signer is often run in environments where logs are piped to stdout like Docker Compose, having a built-in JSON layout option would significantly simplify the deployment pipeline. Even a basic 'standard' JSON output would cover the majority of use cases.

Since you already support Event templates, it would be great to add ECS or GCP templates as selectable options via a --log-format CLI flag.