[Java] Support userloggers.

[jovany-wang]

Why are these changes needed?

In this PR, we support the ability that users can configure their custom loggers with name, logger file name and pattern.

To enable this, you can configure them in your ray.conf as following example:

ray {
logging {

    loggers: [
      {
         name: "userlogger1"
         file-name: "userlogger1"
         pattern: "%d{yyyy-MM-dd HH:mm:ss,SSS} %p %c{1} [%t]: %m%n"
      }
      {
         name: "userlogger2"
         file-name: "userlogger2"
         pattern: "%d{yyyy-MM-dd HH:mm:ss,SSS} %p %c{1} [%t]: %m%n"
      }
    ]

}
}

And then, in your Ray application code, you can get the logger by the name you defined in the config:

Logger LOG = LoggerFactory.getLogger("userlogger1");
LOG.info("This is a custom log.");       // This log will be printed to the log file userlogger1.log.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[jovany-wang]

Let's see the CI result.

[kfstorm]

Windows CI failure is unrelated.