New pulsar source

[jszwedko]

Suggested by user in discord.

[jaysonsantos]

Relaying the chat message for better visibility here.

Hey there folks, I'd like to contribute with vector by adding a pulsar source but I got a few questions first, do you mind shedding some light?
I'd like to try and build it with at-most-once guarantees and being re-entrant and to do that with s3 sink for example, the flow i was thinking is:
- use exclusive/failover subscription so batching is more efficient and i am able to acknowledge the last message so pulsar makes the atomic commit
- send messages to the sink with batching notifier
- acknowledge only the last message which the sink managed to push to the final destination
and my questions about it are:
- i saw that kafka source uses batch notifier [1] which calls the receiver function for every log entry, is it feasible to aggregate the coming messages from a few seconds in order to ack only the last one?
- the receiver would be called only when the s3 sink actually put the file into the bucket?
- could the sink be configured to use the datetime of the first message on the batch, so if the worker dies before acknowledging pulsar it can be re-entrant by pushing the batch file with the same name (at the expense of upload bandwidth)?

[1] https://github.com/timberio/vector/blob/master/src/sources/kafka.rs#L215-L219

[sumeet-zuora]

@jaysonsantos were you able to fork and create pulsar source? we are also interested

[bwmcadams]

I am working on a Pulsar source for Vector, hopefully to have a PR soonish.

[bmcadams-datastax]

I am working on a Pulsar source for Vector, hopefully to have a PR soonish.

scratch that as it looks like there's an open PR for it already