rpk/transform/deploy: support deploying to multiple output topics

[rockwotj]

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x

Release Notes

Features

• rpk transform deploy now supports multiple output topics

[r-vasquez]

LGTM 👍

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/45824#018e1ad7-32cb-43ec-b800-5e13e99e3085

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/45824#018e1aff-8d4f-4833-947e-7329888fad17

[rockwotj]

Force push: revert changing the flag name. @r-vasquez do you have a recommendation for this? The latest push you can specify --output-topic multiple times, but you can't do --output-topic=foo,bar, because I think --output-topics would be a better flag name for that behavior. Options:

1. Force config file usage for multiple output topics (no flag changes).
2. Have both flags --output-topic and --output-topics and they just append to the same slice
   In this model --output-topic could only be specified once, and --output-topics would be
   StringSliceVarP
3. Breaking change to --output-topics (and make it StringSliceVarP)
4. This PR - just allow --output-topic multiple times (StringArrayVarP)

[r-vasquez]

@rockwotj Would StringSliceVar work here (keeping the name output-topic in singular)? it let you repeat and provide comma-separated values.

https://pkg.go.dev/github.com/spf13/pflag#StringSliceVar

We do that already in rpk, for example rpk acl delete:

      --topic strings                  Topic to remove ACLs for (repeatable)

We make sure to put (repeatable) in the help text though

[rockwotj]

Ah nice, yes that works for me thanks! (and done)