Minor issue with slack_post component in Redpanda Connect.

Very low priority, no real impact on me at the moment, just something I noticed that might count as a bug.

When using the "slack_post" output component in a Redpanda Connect pipeline, my intention was to use this YAML in the output stanza:

slack_post:
    bot_token: "${secrets.SLACK_APP_BOT_TOKEN}"

Unfortunately, it looks like there is some kind of validation testing going on, because trying to create the pipeline returns an error stating that the "bot_token" value must start with "xoxb-". Although that's technically true (all Slack bot tokens that I've seen are in fact prefixed with "xoxb-") it does mean that I can't include that part of the token in the secret -- I have to drop the prefix from the secret value and then code the pipeline YAML as:

slack_post:
    bot_token: "xoxb-${secrets.SLACK_APP_BOT_TOKEN}"

Not a big deal, and I can leave comments/documentation explaining the issue, but it's the kind of thing that will be confusing to the next person after me who has to maintain the code and/or the app in production, so it might be better to adjust the component validation rules to allow the prior syntax above.

As a quick guess, the fix would be implemented on or about this line of code:

https://github.com/redpanda-data/connect/blob/9f8f9943dc0a5d19384251ac54252a6e1e093f90/internal/impl/slack/output_post.go#L43

Start at internal/impl/slack/output_post.go around line 43 and inspect the slack_post bot_token validation. Verify how secret substitutions are handled, then adjust the validation so a complete token supplied through ${secrets.SLACK_APP_BOT_TOKEN} is accepted. Done means both the full-token and prefixed pipeline forms behave correctly.