Add configuration for server to store access logs

[frsyuki]

Add a systemConfig to store access log files to digdag-server. Maybe like server.access-log.path = <path>.

[frsyuki]

io.digdag.server.ServerBootstrap will need to wrap PathHandler in AccessLogHandler if server.access-log.path is set.

• JavaDoc: http://undertow.io/javadoc/1.2.x/io/undertow/server/handlers/accesslog/package-summary.html
• Constructor of AccessLogHandler: http://undertow.io/javadoc/1.2.x/io/undertow/server/handlers/accesslog/AccessLogHandler.html#AccessLogHandler-io.undertow.server.HttpHandler-io.undertow.server.handlers.accesslog.AccessLogReceiver-java.lang.String-java.lang.ClassLoader-
• Source: https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/server/handlers/accesslog/AccessLogHandler.java

[frsyuki]

we want to have an additional field (maybe JSON) so that Authenticator can include site_id or some other info to the logs.

[frsyuki]

683edd2d3189ad909b30fd7a05e44a311e9ada39 added --access-log option but it's not configurable and not extensible.

[frsyuki]

now digdag supports server.access-log.pattern option and json log format. json log format can include site_id, and other custom fields added by applications.