Vector: New `fluent` source

[binarylogic]

This will make it easier for users to transition in production environments. We basically want to support the forward input and output for Fluentbit. I believe it's the same protocol used for FluentD, but we should investigate further. They have a section in their docs that covers this.

• [ ] Add a Fluent transitioning guide that walks through how to do this.

[snemetz]

There are more use cases than just transition.
We use AWS fargate which can log to CloudWatch or Fluentd/Fluentbit. Replacing that with vector is not currently an option.
But I'd love to be able to have all of them send to a vector aggregator instead of a fluentd one

[binarylogic]

Thanks, @snemetz. I agree, and we'll prioritize this.