Add MessageContext reference to the DataUnit

[gayaldassanayake]

Purpose

Needed for wso2/product-micro-integrator#4050

Goals

To extend the analytics and allow custom properties to be added to analytics, the users must be allowed to implement an interface provided in product-micro-integrator repository. We need to pass the MessageContext to the user so that they can generate important params to the analytics.

However, ATM we only pass a data unit that contains a subset of the message context properties.

Approach

Add the message context into the data unit using a setter, at the time of creation of the data unit

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