[fix][fn] Go functions must retrieve consumers by non-particioned topic ID

[flowchartsman]

Fixes apache#19367

Modifications

Retrieve consumers from the index based off of non-partitioned topic name.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Note: there is no e2e for Go pulsar functions. Verifying this change is a matter of not having panics when using pulsar functions with partitioned topics.

Does this pull request potentially affect one of the following parts:

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• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: #3