Improve log mediator to support string templating

[SanojPunchihewa]

Purpose

In the current log mediator, the user needs to add a property and use XPATH concatenations to define a log message. With this improvement, the user can define a string template for the log message. In addition to the log message, the user can add additional properties to be logged.

<log category="INFO">
    <message>Test clone path 1 "logging" ${var.var1}</message>
</log>

<log category="INFO">
    <message>Test message "logging" ${var.var1}</message>
    <property name="var2" value="test-prop1"/>
    <property name="var3" expression="$ctx:TOKEN_HEADER"/>
</log>