Show consumer count column on Mgmt UI Channels page (backport #13198)

[mergify]

Proposed Changes

Consumer count is already returned by the /channels API endpoint.
Now the consumer count column can be shown in the channels table but it is hidden by default.

For example in case the global consumer count is unexpectedly high it is useful to see which channels have the most consumers or how (un)evenly they are distributed between channels.

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This is an automatic backport of pull request #13198 done by [Mergify](https://mergify.com).