Add Inline synapse expression support for Payload Factory mediator

[SanojPunchihewa]

Purpose

Add Inline synapse expression support for Payload Factory mediator. With this improvement we can use the new synapse expressions in the template itself as follows,

<payloadFactory media-type="json">
    <format><![CDATA[{
        "variable": ${var.id},
        "username": "${payload.customer_name}",
        "escapedVariable": "${var.id}",
        "bool": ${payload.path},
        "escapedBool": "${payload.path}",
        "escapedObject": "${payload.pet}",
        "object": ${payload.pet}
        }]]>
        </format>
<args/>
</payloadFactory>

Related Issue

wso2/product-micro-integrator#3625