Add variable support for call mediator and improve synapse expression

[arunans23]

Purpose

Describe the problems, issues, or needs driving this feature/fix and include links to related issues in the following format: Resolves issue1, issue2, etc.

This PR adds variable support to Call mediator target so that the response elements (payload, headers and attributes) can be stored in a variable.

          <call>
            <endpoint>
               <http method="POST" uri-template="https://run.mocky.io/v3/fc50337e-e54d-45a7-8bc8-f5c1b53b2519"/>
            </endpoint>
            <target type="variable">{${params.funcParams.targetVariable}}</target>
         </call>

The Synapse expression has been improved to handle Map type variables so that the above stored response elements can be accessed.

Eg:

         <log level="simple">
            <property name="messagenew" expression="${var.resultMocky['payload']}"/>
            <property name="statusCode" expression="${var.resultMocky['attributes'].statusCode}"></property>
            <property name="content-length" expression="${var.resultMocky['headers']['Content-Length']}"></property>
         </log>

[SanojPunchihewa]

If we get a requirement to enrich to variables in other mediator, this may lead to a conflict. WDYT?

[arunans23]

Since we are not giving priority to the Enrich mediator going forward, we don't have to consider this.