Add ability to seek from current position in Reader interface

**Is your enhancement request related to a problem? Please describe.**
The Reader interface has the ability to seek to an arbitrary timestamp, MessageId, first or last message in a topic, or to read the next message. However, there is no API to seek fowards/backwards from the current position in a topic.

Ours I'm sure is not a common use-case, but you might imagine other situations where this could be useful. Ours is that we originally used Kafka for our pub/sub system and implemented cluster-wide logging via Kafka's Log4jAppender. We tried to implement the same thing using Pulsar, but we are unable to request the last _n_ logs because we can't seek backwards _n_ messages from the end of the topic. (Requesting logs from a particular timestamp onward was much easier to implement than in Kafka though!)

**Describe the solution you'd like**
Add a method to seek backwards (and forwards) from the current position by a number of messages.

**Describe alternatives you've considered**
**Edit:** See the workaround in my comment below.
~~I tried finding the last available MessageId and then manipulating the fields. Decrementing the MessageIdImpl.entryId seems to work, but only to the beginning of the ledger. I could also decrement the ledgerId, but I wouldn't know the Ledger's top entryId to start with. Plus, I'm not sure this is always a safe thing to do!~~

Start with the Reader interface and its existing seek APIs, then review the issue's comment thread, including the documented workaround. Define how seeking forward and backward by a message count should behave across topic boundaries before implementing it, and verify the behavior with the relevant Reader tests.

Written by the indexing model from the issue text.