Provide a parameterless CustomRequestLog

[sbordet]

Jetty version
10.0.x

Description
Currently, CustomRequestLog requires constructor parameters.
This does not play well with the Jetty Maven Plugin, which expects a RequestLog instance, but it's going to be difficult to instantiate from a pom.xml file.

While it is true that custom RequestLog configurations may be provided with a Jetty XML file, simple usages will benefit from a parameterless CustomRequestLog.

Example in CometD 6:

<plugin>
  <groupId>org.eclipse.jetty</groupId>
  <artifactId>jetty-maven-plugin</artifactId>
  <configuration>
    <requestLog implementation="org.eclipse.jetty.server.Slf4jRequestLog" /> // Present in Jetty 9.4.x, but not in 10.0.x. 
    ...
  </configuration>
</plugin>

[sbordet]

@janbartel opinions about this?