Binder preserves headers on send/receive

[onobc]

• Refactor the existing PulsarMessageHeaderMapper to include to/from support.
• Use the new API in the binder send/receive
• Add tests for header mapper

See #358

[onobc]

Complex header values

• Spring MessageHeaders have Object values
• Pulsar message properties have String values

In the 1st commit I simple called toString on the send side. Interestingly enough, this was sufficient for fields such as UID and Timestamp. The message header code must be doing some smarts via string serialization. @sobychacko can you confirm this?

[sobychacko]

@onobc When we added the Pulsar headers support to the regular listener, I don't think we added anything in terms of serialization. We can chat further about it offline.

[onobc]

TODO

• in subsequent PR need to add support for complex header types (see DefaultKafkaHeaderMapper for inspiration)
• in subsequent PR need to add support for filtering what headers are considered (see DefaultKafkaHeaderMapper for inspiration)
• in subsequent PR make mapper auto-configured and user extendable