Adding support to extract values from message header.

[kishoreg]

Most stream systems have started having an envelope over the actual message. For e.g. Kafka has headers and pubsub supports attributes.

Today, we dont have a way to extract values from headers. This PR adds support for these headers.

Right now it's only supporting Kafka and leverages the existing rowmetadata implementation. I am trying to keep the PR simple. let me know if there is a better way to support this.

Will add test case shortly

Instructions:

1. The PR has to be tagged with at least one of the following labels (*):
   1. feature
   2. bugfix
   3. performance
   4. ui
   5. backward-incompat
   6. release-notes (**)
2. Remove these instructions before publishing the PR.

(*) Other labels to consider:

• testing
• dependencies
• docker
• kubernetes
• observability
• security
• code-style
• extension-point
• refactor
• cleanup

(**) Use release-notes label for scenarios like:

• New configuration options
• Deprecation of configurations
• Signature changes to public methods/interfaces
• New plugins added or old plugins removed