[source-slack] Channel filtering: wildcard support, exclude list, external-channel toggle

> 🤖 *This issue was generated by an AI Agent.*

## What's missing

`channel_filter` matches channel names exactly. There is no way to match by pattern, to exclude channels, or to include/exclude externally-shared channels.

## Verified on `master`

`airbyte-integrations/connectors/source-slack/manifest.yaml` line 221 filters on exact membership:

```yaml
condition: "{{ record.name in config.channel_filter or not config.channel_filter }}"
```

`is_ext_shared` exists only as a record schema field, not as a config control.

## Origin

Closed as stale: #74321 (@hlepouse-vasco), reviewed 2026-04-16 with no author response.

Filed to keep a record of the gap. The PR was closed for inactivity, not on merit — credit to the original contributor if this is picked up.

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13179