Add fluent API for PulsarTemplate

[onobc]

This will avoid API signature permutation explosion as we extend the API in the future, thus avoiding a maintenance headache.

We will offer a hybrid API that consists of the following:

• Keep the following existing non-fluent API for simple use cases:

  send(message)
  send(topic, message)
  sendAsync(message)
  sendAsnyc(topic, message)

• Replace all other variants w/ a fluent based DSL as such:

  template.prepare()
      .message("5150")
      .toTopic("topic-one")
      //.toDefaultTopic()
      .withRouter(customRouter)
      .withInterceptor(customInterceptor)
      .withCustomizer(messageKeyCustomizer)
      .send(); // .sendAsync()

Points to consider

• This will lend itself well to the chunking/batching configuration enhancements (Configurable retry mechanism in PulsarTemplate sendAsync #16)
• Ensure the design accommodates template extensions such as (Explore the option of routable Pulsar template #67)
• What is the impact to PulsarOperations? Should we keep it?