feat: transactions

[rkrishn7]

This PR adds client support for Pulsar Transactions.

Resolves #253

Notes:

• This PR tries to mimic the Java client where possible. More details on transactions in Pulsar can be found here.
• Will likely need to edit the Pulsar service container in CI to enable the TC and bootstrap transaction metadata
• This PR necessitates a breaking version change as it adds a new error variant

Open Questions:

• Should these changes be gated behind a feature flag?
• This change likely has implications when buffering in the producer. Thoughts on how we should handle that?
• The main Transaction API uses internal synchronization to provide a "no mut" public interface. Are there any alternative approaches that may be better?

[rkrishn7]

@FlorentinDUBOIS Tagging you for visibility. I know this is a lot so no rush here!

[rkrishn7]

Bumping and tagging some additional folks - @freeznet @BewareMyPower